Documentation / block / biodoc.txt

Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

		Notes on the Generic Block Layer Rewrite in Linux 2.5
		=====================================================
	
	Notes Written on Jan 15, 2002:
		Jens Axboe <jens.axboe@oracle.com>
		Suparna Bhattacharya <suparna@in.ibm.com>
	
	Last Updated May 2, 2002
	September 2003: Updated I/O Scheduler portions
		Nick Piggin <npiggin@kernel.dk>
	
	Introduction:
	
	These are some notes describing some aspects of the 2.5 block layer in the
	context of the bio rewrite. The idea is to bring out some of the key
	changes and a glimpse of the rationale behind those changes.
	
	Please mail corrections & suggestions to suparna@in.ibm.com.
	
	Credits:
	---------
	
	2.5 bio rewrite:
		Jens Axboe <jens.axboe@oracle.com>
	
	Many aspects of the generic block layer redesign were driven by and evolved
	over discussions, prior patches and the collective experience of several
	people. See sections 8 and 9 for a list of some related references.
	
	The following people helped with review comments and inputs for this
	document:
		Christoph Hellwig <hch@infradead.org>
		Arjan van de Ven <arjanv@redhat.com>
		Randy Dunlap <rdunlap@xenotime.net>
		Andre Hedrick <andre@linux-ide.org>
	
	The following people helped with fixes/contributions to the bio patches
	while it was still work-in-progress:
		David S. Miller <davem@redhat.com>
	
	
	Description of Contents:
	------------------------
	
	1. Scope for tuning of logic to various needs
	  1.1 Tuning based on device or low level driver capabilities
		- Per-queue parameters
		- Highmem I/O support
		- I/O scheduler modularization
	  1.2 Tuning based on high level requirements/capabilities
		1.2.1 Request Priority/Latency
	  1.3 Direct access/bypass to lower layers for diagnostics and special
	      device operations
		1.3.1 Pre-built commands
	2. New flexible and generic but minimalist i/o structure or descriptor
	   (instead of using buffer heads at the i/o layer)
	  2.1 Requirements/Goals addressed
	  2.2 The bio struct in detail (multi-page io unit)
	  2.3 Changes in the request structure
	3. Using bios
	  3.1 Setup/teardown (allocation, splitting)
	  3.2 Generic bio helper routines
	    3.2.1 Traversing segments and completion units in a request
	    3.2.2 Setting up DMA scatterlists
	    3.2.3 I/O completion
	    3.2.4 Implications for drivers that do not interpret bios (don't handle
	 	  multiple segments)
	    3.2.5 Request command tagging
	  3.3 I/O submission
	4. The I/O scheduler
	5. Scalability related changes
	  5.1 Granular locking: Removal of io_request_lock
	  5.2 Prepare for transition to 64 bit sector_t
	6. Other Changes/Implications
	  6.1 Partition re-mapping handled by the generic block layer
	7. A few tips on migration of older drivers
	8. A list of prior/related/impacted patches/ideas
	9. Other References/Discussion Threads
	
	---------------------------------------------------------------------------
	
	Bio Notes
	--------
	
	Let us discuss the changes in the context of how some overall goals for the
	block layer are addressed.
	
	1. Scope for tuning the generic logic to satisfy various requirements
	
	The block layer design supports adaptable abstractions to handle common
	processing with the ability to tune the logic to an appropriate extent
	depending on the nature of the device and the requirements of the caller.
	One of the objectives of the rewrite was to increase the degree of tunability
	and to enable higher level code to utilize underlying device/driver
	capabilities to the maximum extent for better i/o performance. This is
	important especially in the light of ever improving hardware capabilities
	and application/middleware software designed to take advantage of these
	capabilities.
	
	1.1 Tuning based on low level device / driver capabilities
	
	Sophisticated devices with large built-in caches, intelligent i/o scheduling
	optimizations, high memory DMA support, etc may find some of the
	generic processing an overhead, while for less capable devices the
	generic functionality is essential for performance or correctness reasons.
	Knowledge of some of the capabilities or parameters of the device should be
	used at the generic block layer to take the right decisions on
	behalf of the driver.
	
	How is this achieved ?
	
	Tuning at a per-queue level:
	
	i. Per-queue limits/values exported to the generic layer by the driver
	
	Various parameters that the generic i/o scheduler logic uses are set at
	a per-queue level (e.g maximum request size, maximum number of segments in
	a scatter-gather list, logical block size)
	
	Some parameters that were earlier available as global arrays indexed by
	major/minor are now directly associated with the queue. Some of these may
	move into the block device structure in the future. Some characteristics
	have been incorporated into a queue flags field rather than separate fields
	in themselves.  There are blk_queue_xxx functions to set the parameters,
	rather than update the fields directly
	
	Some new queue property settings:
	
		blk_queue_bounce_limit(q, u64 dma_address)
			Enable I/O to highmem pages, dma_address being the
			limit. No highmem default.
	
		blk_queue_max_sectors(q, max_sectors)
			Sets two variables that limit the size of the request.
	
			- The request queue's max_sectors, which is a soft size in
			units of 512 byte sectors, and could be dynamically varied
			by the core kernel.
	
			- The request queue's max_hw_sectors, which is a hard limit
			and reflects the maximum size request a driver can handle
			in units of 512 byte sectors.
	
			The default for both max_sectors and max_hw_sectors is
			255. The upper limit of max_sectors is 1024.
	
		blk_queue_max_phys_segments(q, max_segments)
			Maximum physical segments you can handle in a request. 128
			default (driver limit). (See 3.2.2)
	
		blk_queue_max_hw_segments(q, max_segments)
			Maximum dma segments the hardware can handle in a request. 128
			default (host adapter limit, after dma remapping).
			(See 3.2.2)
	
		blk_queue_max_segment_size(q, max_seg_size)
			Maximum size of a clustered segment, 64kB default.
	
		blk_queue_logical_block_size(q, logical_block_size)
			Lowest possible sector size that the hardware can operate
			on, 512 bytes default.
	
	New queue flags:
	
		QUEUE_FLAG_CLUSTER (see 3.2.2)
		QUEUE_FLAG_QUEUED (see 3.2.4)
	
	
	ii. High-mem i/o capabilities are now considered the default
	
	The generic bounce buffer logic, present in 2.4, where the block layer would
	by default copyin/out i/o requests on high-memory buffers to low-memory buffers
	assuming that the driver wouldn't be able to handle it directly, has been
	changed in 2.5. The bounce logic is now applied only for memory ranges
	for which the device cannot handle i/o. A driver can specify this by
	setting the queue bounce limit for the request queue for the device
	(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
	where a device is capable of handling high memory i/o.
	
	In order to enable high-memory i/o where the device is capable of supporting
	it, the pci dma mapping routines and associated data structures have now been
	modified to accomplish a direct page -> bus translation, without requiring
	a virtual address mapping (unlike the earlier scheme of virtual address
	-> bus translation). So this works uniformly for high-memory pages (which
	do not have a corresponding kernel virtual address space mapping) and
	low-memory pages.
	
	Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion
	on PCI high mem DMA aspects and mapping of scatter gather lists, and support
	for 64 bit PCI.
	
	Special handling is required only for cases where i/o needs to happen on
	pages at physical memory addresses beyond what the device can support. In these
	cases, a bounce bio representing a buffer from the supported memory range
	is used for performing the i/o with copyin/copyout as needed depending on
	the type of the operation.  For example, in case of a read operation, the
	data read has to be copied to the original buffer on i/o completion, so a
	callback routine is set up to do this, while for write, the data is copied
	from the original buffer to the bounce buffer prior to issuing the
	operation. Since an original buffer may be in a high memory area that's not
	mapped in kernel virtual addr, a kmap operation may be required for
	performing the copy, and special care may be needed in the completion path
	as it may not be in irq context. Special care is also required (by way of
	GFP flags) when allocating bounce buffers, to avoid certain highmem
	deadlock possibilities.
	
	It is also possible that a bounce buffer may be allocated from high-memory
	area that's not mapped in kernel virtual addr, but within the range that the
	device can use directly; so the bounce page may need to be kmapped during
	copy operations. [Note: This does not hold in the current implementation,
	though]
	
	There are some situations when pages from high memory may need to
	be kmapped, even if bounce buffers are not necessary. For example a device
	may need to abort DMA operations and revert to PIO for the transfer, in
	which case a virtual mapping of the page is required. For SCSI it is also
	done in some scenarios where the low level driver cannot be trusted to
	handle a single sg entry correctly. The driver is expected to perform the
	kmaps as needed on such occasions as appropriate. A driver could also use
	the blk_queue_bounce() routine on its own to bounce highmem i/o to low
	memory for specific requests if so desired.
	
	iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
	
	As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
	queue or pick from (copy) existing generic schedulers and replace/override
	certain portions of it. The 2.5 rewrite provides improved modularization
	of the i/o scheduler. There are more pluggable callbacks, e.g for init,
	add request, extract request, which makes it possible to abstract specific
	i/o scheduling algorithm aspects and details outside of the generic loop.
	It also makes it possible to completely hide the implementation details of
	the i/o scheduler from block drivers.
	
	I/O scheduler wrappers are to be used instead of accessing the queue directly.
	See section 4. The I/O scheduler for details.
	
	1.2 Tuning Based on High level code capabilities
	
	i. Application capabilities for raw i/o
	
	This comes from some of the high-performance database/middleware
	requirements where an application prefers to make its own i/o scheduling
	decisions based on an understanding of the access patterns and i/o
	characteristics
	
	ii. High performance filesystems or other higher level kernel code's
	capabilities
	
	Kernel components like filesystems could also take their own i/o scheduling
	decisions for optimizing performance. Journalling filesystems may need
	some control over i/o ordering.
	
	What kind of support exists at the generic block layer for this ?
	
	The flags and rw fields in the bio structure can be used for some tuning
	from above e.g indicating that an i/o is just a readahead request, or priority
	settings (currently unused). As far as user applications are concerned they
	would need an additional mechanism either via open flags or ioctls, or some
	other upper level mechanism to communicate such settings to block.
	
	1.2.1 Request Priority/Latency
	
	Todo/Under discussion:
	Arjan's proposed request priority scheme allows higher levels some broad
	  control (high/med/low) over the priority  of an i/o request vs other pending
	  requests in the queue. For example it allows reads for bringing in an
	  executable page on demand to be given a higher priority over pending write
	  requests which haven't aged too much on the queue. Potentially this priority
	  could even be exposed to applications in some manner, providing higher level
	  tunability. Time based aging avoids starvation of lower priority
	  requests. Some bits in the bi_opf flags field in the bio structure are
	  intended to be used for this priority information.
	
	
	1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
	    (e.g Diagnostics, Systems Management)
	
	There are situations where high-level code needs to have direct access to
	the low level device capabilities or requires the ability to issue commands
	to the device bypassing some of the intermediate i/o layers.
	These could, for example, be special control commands issued through ioctl
	interfaces, or could be raw read/write commands that stress the drive's
	capabilities for certain kinds of fitness tests. Having direct interfaces at
	multiple levels without having to pass through upper layers makes
	it possible to perform bottom up validation of the i/o path, layer by
	layer, starting from the media.
	
	The normal i/o submission interfaces, e.g submit_bio, could be bypassed
	for specially crafted requests which such ioctl or diagnostics
	interfaces would typically use, and the elevator add_request routine
	can instead be used to directly insert such requests in the queue or preferably
	the blk_do_rq routine can be used to place the request on the queue and
	wait for completion. Alternatively, sometimes the caller might just
	invoke a lower level driver specific interface with the request as a
	parameter.
	
	If the request is a means for passing on special information associated with
	the command, then such information is associated with the request->special
	field (rather than misuse the request->buffer field which is meant for the
	request data buffer's virtual mapping).
	
	For passing request data, the caller must build up a bio descriptor
	representing the concerned memory buffer if the underlying driver interprets
	bio segments or uses the block layer end*request* functions for i/o
	completion. Alternatively one could directly use the request->buffer field to
	specify the virtual address of the buffer, if the driver expects buffer
	addresses passed in this way and ignores bio entries for the request type
	involved. In the latter case, the driver would modify and manage the
	request->buffer, request->sector and request->nr_sectors or
	request->current_nr_sectors fields itself rather than using the block layer
	end_request or end_that_request_first completion interfaces.
	(See 2.3 or Documentation/block/request.txt for a brief explanation of
	the request structure fields)
	
	[TBD: end_that_request_last should be usable even in this case;
	Perhaps an end_that_direct_request_first routine could be implemented to make
	handling direct requests easier for such drivers; Also for drivers that
	expect bios, a helper function could be provided for setting up a bio
	corresponding to a data buffer]
	
	<JENS: I dont understand the above, why is end_that_request_first() not
	usable? Or _last for that matter. I must be missing something>
	<SUP: What I meant here was that if the request doesn't have a bio, then
	 end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
	 and hence can't be used for advancing request state settings on the
	 completion of partial transfers. The driver has to modify these fields 
	 directly by hand.
	 This is because end_that_request_first only iterates over the bio list,
	 and always returns 0 if there are none associated with the request.
	 _last works OK in this case, and is not a problem, as I mentioned earlier
	>
	
	1.3.1 Pre-built Commands
	
	A request can be created with a pre-built custom command  to be sent directly
	to the device. The cmd block in the request structure has room for filling
	in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
	command pre-building, and the type of the request is now indicated
	through rq->flags instead of via rq->cmd)
	
	The request structure flags can be set up to indicate the type of request
	in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
	packet command issued via blk_do_rq, REQ_SPECIAL: special request).
	
	It can help to pre-build device commands for requests in advance.
	Drivers can now specify a request prepare function (q->prep_rq_fn) that the
	block layer would invoke to pre-build device commands for a given request,
	or perform other preparatory processing for the request. This is routine is
	called by elv_next_request(), i.e. typically just before servicing a request.
	(The prepare function would not be called for requests that have RQF_DONTPREP
	enabled)
	
	Aside:
	  Pre-building could possibly even be done early, i.e before placing the
	  request on the queue, rather than construct the command on the fly in the
	  driver while servicing the request queue when it may affect latencies in
	  interrupt context or responsiveness in general. One way to add early
	  pre-building would be to do it whenever we fail to merge on a request.
	  Now REQ_NOMERGE is set in the request flags to skip this one in the future,
	  which means that it will not change before we feed it to the device. So
	  the pre-builder hook can be invoked there.
	
	
	2. Flexible and generic but minimalist i/o structure/descriptor.
	
	2.1 Reason for a new structure and requirements addressed
	
	Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
	layer, and the low level request structure was associated with a chain of
	buffer heads for a contiguous i/o request. This led to certain inefficiencies
	when it came to large i/o requests and readv/writev style operations, as it
	forced such requests to be broken up into small chunks before being passed
	on to the generic block layer, only to be merged by the i/o scheduler
	when the underlying device was capable of handling the i/o in one shot.
	Also, using the buffer head as an i/o structure for i/os that didn't originate
	from the buffer cache unnecessarily added to the weight of the descriptors
	which were generated for each such chunk.
	
	The following were some of the goals and expectations considered in the
	redesign of the block i/o data structure in 2.5.
	
	i.  Should be appropriate as a descriptor for both raw and buffered i/o  -
	    avoid cache related fields which are irrelevant in the direct/page i/o path,
	    or filesystem block size alignment restrictions which may not be relevant
	    for raw i/o.
	ii. Ability to represent high-memory buffers (which do not have a virtual
	    address mapping in kernel address space).
	iii.Ability to represent large i/os w/o unnecessarily breaking them up (i.e
	    greater than PAGE_SIZE chunks in one shot)
	iv. At the same time, ability to retain independent identity of i/os from
	    different sources or i/o units requiring individual completion (e.g. for
	    latency reasons)
	v.  Ability to represent an i/o involving multiple physical memory segments
	    (including non-page aligned page fragments, as specified via readv/writev)
	    without unnecessarily breaking it up, if the underlying device is capable of
	    handling it.
	vi. Preferably should be based on a memory descriptor structure that can be
	    passed around different types of subsystems or layers, maybe even
	    networking, without duplication or extra copies of data/descriptor fields
	    themselves in the process
	vii.Ability to handle the possibility of splits/merges as the structure passes
	    through layered drivers (lvm, md, evms), with minimal overhead.
	
	The solution was to define a new structure (bio)  for the block layer,
	instead of using the buffer head structure (bh) directly, the idea being
	avoidance of some associated baggage and limitations. The bio structure
	is uniformly used for all i/o at the block layer ; it forms a part of the
	bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
	mapped to bio structures.
	
	2.2 The bio struct
	
	The bio structure uses a vector representation pointing to an array of tuples
	of <page, offset, len> to describe the i/o buffer, and has various other
	fields describing i/o parameters and state that needs to be maintained for
	performing the i/o.
	
	Notice that this representation means that a bio has no virtual address
	mapping at all (unlike buffer heads).
	
	struct bio_vec {
	       struct page     *bv_page;
	       unsigned short  bv_len;
	       unsigned short  bv_offset;
	};
	
	/*
	 * main unit of I/O for the block layer and lower layers (ie drivers)
	 */
	struct bio {
	       struct bio          *bi_next;    /* request queue link */
	       struct block_device *bi_bdev;	/* target device */
	       unsigned long       bi_flags;    /* status, command, etc */
	       unsigned long       bi_opf;       /* low bits: r/w, high: priority */
	
	       unsigned int	bi_vcnt;     /* how may bio_vec's */
	       struct bvec_iter	bi_iter;	/* current index into bio_vec array */
	
	       unsigned int	bi_size;     /* total size in bytes */
	       unsigned short 	bi_phys_segments; /* segments after physaddr coalesce*/
	       unsigned short	bi_hw_segments; /* segments after DMA remapping */
	       unsigned int	bi_max;	     /* max bio_vecs we can hold
	                                        used as index into pool */
	       struct bio_vec   *bi_io_vec;  /* the actual vec list */
	       bio_end_io_t	*bi_end_io;  /* bi_end_io (bio) */
	       atomic_t		bi_cnt;	     /* pin count: free when it hits zero */
	       void             *bi_private;
	};
	
	With this multipage bio design:
	
	- Large i/os can be sent down in one go using a bio_vec list consisting
	  of an array of <page, offset, len> fragments (similar to the way fragments
	  are represented in the zero-copy network code)
	- Splitting of an i/o request across multiple devices (as in the case of
	  lvm or raid) is achieved by cloning the bio (where the clone points to
	  the same bi_io_vec array, but with the index and size accordingly modified)
	- A linked list of bios is used as before for unrelated merges (*) - this
	  avoids reallocs and makes independent completions easier to handle.
	- Code that traverses the req list can find all the segments of a bio
	  by using rq_for_each_segment.  This handles the fact that a request
	  has multiple bios, each of which can have multiple segments.
	- Drivers which can't process a large bio in one shot can use the bi_iter
	  field to keep track of the next bio_vec entry to process.
	  (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
	  [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
	   bi_offset an len fields]
	
	(*) unrelated merges -- a request ends up containing two or more bios that
	    didn't originate from the same place.
	
	bi_end_io() i/o callback gets called on i/o completion of the entire bio.
	
	At a lower level, drivers build a scatter gather list from the merged bios.
	The scatter gather list is in the form of an array of <page, offset, len>
	entries with their corresponding dma address mappings filled in at the
	appropriate time. As an optimization, contiguous physical pages can be
	covered by a single entry where <page> refers to the first page and <len>
	covers the range of pages (up to 16 contiguous pages could be covered this
	way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
	the sg list.
	
	Note: Right now the only user of bios with more than one page is ll_rw_kio,
	which in turn means that only raw I/O uses it (direct i/o may not work
	right now). The intent however is to enable clustering of pages etc to
	become possible. The pagebuf abstraction layer from SGI also uses multi-page
	bios, but that is currently not included in the stock development kernels.
	The same is true of Andrew Morton's work-in-progress multipage bio writeout 
	and readahead patches.
	
	2.3 Changes in the Request Structure
	
	The request structure is the structure that gets passed down to low level
	drivers. The block layer make_request function builds up a request structure,
	places it on the queue and invokes the drivers request_fn. The driver makes
	use of block layer helper routine elv_next_request to pull the next request
	off the queue. Control or diagnostic functions might bypass block and directly
	invoke underlying driver entry points passing in a specially constructed
	request structure.
	
	Only some relevant fields (mainly those which changed or may be referred
	to in some of the discussion here) are listed below, not necessarily in
	the order in which they occur in the structure (see include/linux/blkdev.h)
	Refer to Documentation/block/request.txt for details about all the request
	structure fields and a quick reference about the layers which are
	supposed to use or modify those fields.
	
	struct request {
		struct list_head queuelist;  /* Not meant to be directly accessed by
						the driver.
						Used by q->elv_next_request_fn
						rq->queue is gone
						*/
		.
		.
		unsigned char cmd[16]; /* prebuilt command data block */
		unsigned long flags;   /* also includes earlier rq->cmd settings */
		.
		.
		sector_t sector; /* this field is now of type sector_t instead of int
				    preparation for 64 bit sectors */
		.
		.
	
		/* Number of scatter-gather DMA addr+len pairs after
		 * physical address coalescing is performed.
		 */
		unsigned short nr_phys_segments;
	
		/* Number of scatter-gather addr+len pairs after
		 * physical and DMA remapping hardware coalescing is performed.
		 * This is the number of scatter-gather entries the driver
		 * will actually have to deal with after DMA mapping is done.
		 */
		unsigned short nr_hw_segments;
	
		/* Various sector counts */
		unsigned long nr_sectors;  /* no. of sectors left: driver modifiable */
		unsigned long hard_nr_sectors;  /* block internal copy of above */
		unsigned int current_nr_sectors; /* no. of sectors left in the
						   current segment:driver modifiable */
		unsigned long hard_cur_sectors; /* block internal copy of the above */
		.
		.
		int tag;	/* command tag associated with request */
		void *special;  /* same as before */
		char *buffer;   /* valid only for low memory buffers up to
				 current_nr_sectors */
		.
		.
		struct bio *bio, *biotail;  /* bio list instead of bh */
		struct request_list *rl;
	}
		
	See the req_ops and req_flag_bits definitions for an explanation of the various
	flags available. Some bits are used by the block layer or i/o scheduler.
		
	The behaviour of the various sector counts are almost the same as before,
	except that since we have multi-segment bios, current_nr_sectors refers
	to the numbers of sectors in the current segment being processed which could
	be one of the many segments in the current bio (i.e i/o completion unit).
	The nr_sectors value refers to the total number of sectors in the whole
	request that remain to be transferred (no change). The purpose of the
	hard_xxx values is for block to remember these counts every time it hands
	over the request to the driver. These values are updated by block on
	end_that_request_first, i.e. every time the driver completes a part of the
	transfer and invokes block end*request helpers to mark this. The
	driver should not modify these values. The block layer sets up the
	nr_sectors and current_nr_sectors fields (based on the corresponding
	hard_xxx values and the number of bytes transferred) and updates it on
	every transfer that invokes end_that_request_first. It does the same for the
	buffer, bio, bio->bi_iter fields too.
	
	The buffer field is just a virtual address mapping of the current segment
	of the i/o buffer in cases where the buffer resides in low-memory. For high
	memory i/o, this field is not valid and must not be used by drivers.
	
	Code that sets up its own request structures and passes them down to
	a driver needs to be careful about interoperation with the block layer helper
	functions which the driver uses. (Section 1.3)
	
	3. Using bios
	
	3.1 Setup/Teardown
	
	There are routines for managing the allocation, and reference counting, and
	freeing of bios (bio_alloc, bio_get, bio_put).
	
	This makes use of Ingo Molnar's mempool implementation, which enables
	subsystems like bio to maintain their own reserve memory pools for guaranteed
	deadlock-free allocations during extreme VM load. For example, the VM
	subsystem makes use of the block layer to writeout dirty pages in order to be
	able to free up memory space, a case which needs careful handling. The
	allocation logic draws from the preallocated emergency reserve in situations
	where it cannot allocate through normal means. If the pool is empty and it
	can wait, then it would trigger action that would help free up memory or
	replenish the pool (without deadlocking) and wait for availability in the pool.
	If it is in IRQ context, and hence not in a position to do this, allocation
	could fail if the pool is empty. In general mempool always first tries to
	perform allocation without having to wait, even if it means digging into the
	pool as long it is not less that 50% full.
	
	On a free, memory is released to the pool or directly freed depending on
	the current availability in the pool. The mempool interface lets the
	subsystem specify the routines to be used for normal alloc and free. In the
	case of bio, these routines make use of the standard slab allocator.
	
	The caller of bio_alloc is expected to taken certain steps to avoid
	deadlocks, e.g. avoid trying to allocate more memory from the pool while
	already holding memory obtained from the pool.
	[TBD: This is a potential issue, though a rare possibility
	 in the bounce bio allocation that happens in the current code, since
	 it ends up allocating a second bio from the same pool while
	 holding the original bio ]
	
	Memory allocated from the pool should be released back within a limited
	amount of time (in the case of bio, that would be after the i/o is completed).
	This ensures that if part of the pool has been used up, some work (in this
	case i/o) must already be in progress and memory would be available when it
	is over. If allocating from multiple pools in the same code path, the order
	or hierarchy of allocation needs to be consistent, just the way one deals
	with multiple locks.
	
	The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
	for a non-clone bio. There are the 6 pools setup for different size biovecs,
	so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
	given size from these slabs.
	
	The bio_get() routine may be used to hold an extra reference on a bio prior
	to i/o submission, if the bio fields are likely to be accessed after the
	i/o is issued (since the bio may otherwise get freed in case i/o completion
	happens in the meantime).
	
	The bio_clone_fast() routine may be used to duplicate a bio, where the clone
	shares the bio_vec_list with the original bio (i.e. both point to the
	same bio_vec_list). This would typically be used for splitting i/o requests
	in lvm or md.
	
	3.2 Generic bio helper Routines
	
	3.2.1 Traversing segments and completion units in a request
	
	The macro rq_for_each_segment() should be used for traversing the bios
	in the request list (drivers should avoid directly trying to do it
	themselves). Using these helpers should also make it easier to cope
	with block changes in the future.
	
		struct req_iterator iter;
		rq_for_each_segment(bio_vec, rq, iter)
			/* bio_vec is now current segment */
	
	I/O completion callbacks are per-bio rather than per-segment, so drivers
	that traverse bio chains on completion need to keep that in mind. Drivers
	which don't make a distinction between segments and completion units would
	need to be reorganized to support multi-segment bios.
	
	3.2.2 Setting up DMA scatterlists
	
	The blk_rq_map_sg() helper routine would be used for setting up scatter
	gather lists from a request, so a driver need not do it on its own.
	
		nr_segments = blk_rq_map_sg(q, rq, scatterlist);
	
	The helper routine provides a level of abstraction which makes it easier
	to modify the internals of request to scatterlist conversion down the line
	without breaking drivers. The blk_rq_map_sg routine takes care of several
	things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
	is set) and correct segment accounting to avoid exceeding the limits which
	the i/o hardware can handle, based on various queue properties.
	
	- Prevents a clustered segment from crossing a 4GB mem boundary
	- Avoids building segments that would exceed the number of physical
	  memory segments that the driver can handle (phys_segments) and the
	  number that the underlying hardware can handle at once, accounting for
	  DMA remapping (hw_segments)  (i.e. IOMMU aware limits).
	
	Routines which the low level driver can use to set up the segment limits:
	
	blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
	hw data segments in a request (i.e. the maximum number of address/length
	pairs the host adapter can actually hand to the device at once)
	
	blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
	of physical data segments in a request (i.e. the largest sized scatter list
	a driver could handle)
	
	3.2.3 I/O completion
	
	The existing generic block layer helper routines end_request,
	end_that_request_first and end_that_request_last can be used for i/o
	completion (and setting things up so the rest of the i/o or the next
	request can be kicked of) as before. With the introduction of multi-page
	bio support, end_that_request_first requires an additional argument indicating
	the number of sectors completed.
	
	3.2.4 Implications for drivers that do not interpret bios (don't handle
	 multiple segments)
	
	Drivers that do not interpret bios e.g those which do not handle multiple
	segments and do not support i/o into high memory addresses (require bounce
	buffers) and expect only virtually mapped buffers, can access the rq->buffer
	field. As before the driver should use current_nr_sectors to determine the
	size of remaining data in the current segment (that is the maximum it can
	transfer in one go unless it interprets segments), and rely on the block layer
	end_request, or end_that_request_first/last to take care of all accounting
	and transparent mapping of the next bio segment when a segment boundary
	is crossed on completion of a transfer. (The end*request* functions should
	be used if only if the request has come down from block/bio path, not for
	direct access requests which only specify rq->buffer without a valid rq->bio)
	
	3.2.5 Generic request command tagging
	
	3.2.5.1 Tag helpers
	
	Block now offers some simple generic functionality to help support command
	queueing (typically known as tagged command queueing), ie manage more than
	one outstanding command on a queue at any given time.
	
		blk_queue_init_tags(struct request_queue *q, int depth)
	
		Initialize internal command tagging structures for a maximum
		depth of 'depth'.
	
		blk_queue_free_tags((struct request_queue *q)
	
		Teardown tag info associated with the queue. This will be done
		automatically by block if blk_queue_cleanup() is called on a queue
		that is using tagging.
	
	The above are initialization and exit management, the main helpers during
	normal operations are:
	
		blk_queue_start_tag(struct request_queue *q, struct request *rq)
	
		Start tagged operation for this request. A free tag number between
		0 and 'depth' is assigned to the request (rq->tag holds this number),
		and 'rq' is added to the internal tag management. If the maximum depth
		for this queue is already achieved (or if the tag wasn't started for
		some other reason), 1 is returned. Otherwise 0 is returned.
	
		blk_queue_end_tag(struct request_queue *q, struct request *rq)
	
		End tagged operation on this request. 'rq' is removed from the internal
		book keeping structures.
	
	To minimize struct request and queue overhead, the tag helpers utilize some
	of the same request members that are used for normal request queue management.
	This means that a request cannot both be an active tag and be on the queue
	list at the same time. blk_queue_start_tag() will remove the request, but
	the driver must remember to call blk_queue_end_tag() before signalling
	completion of the request to the block layer. This means ending tag
	operations before calling end_that_request_last()! For an example of a user
	of these helpers, see the IDE tagged command queueing support.
	
	Certain hardware conditions may dictate a need to invalidate the block tag
	queue. For instance, on IDE any tagged request error needs to clear both
	the hardware and software block queue and enable the driver to sanely restart
	all the outstanding requests. There's a third helper to do that:
	
		blk_queue_invalidate_tags(struct request_queue *q)
	
		Clear the internal block tag queue and re-add all the pending requests
		to the request queue. The driver will receive them again on the
		next request_fn run, just like it did the first time it encountered
		them.
	
	3.2.5.2 Tag info
	
	Some block functions exist to query current tag status or to go from a
	tag number to the associated request. These are, in no particular order:
	
		blk_queue_tagged(q)
	
		Returns 1 if the queue 'q' is using tagging, 0 if not.
	
		blk_queue_tag_request(q, tag)
	
		Returns a pointer to the request associated with tag 'tag'.
	
		blk_queue_tag_depth(q)
		
		Return current queue depth.
	
		blk_queue_tag_queue(q)
	
		Returns 1 if the queue can accept a new queued command, 0 if we are
		at the maximum depth already.
	
		blk_queue_rq_tagged(rq)
	
		Returns 1 if the request 'rq' is tagged.
	
	3.2.5.2 Internal structure
	
	Internally, block manages tags in the blk_queue_tag structure:
	
		struct blk_queue_tag {
			struct request **tag_index;	/* array or pointers to rq */
			unsigned long *tag_map;		/* bitmap of free tags */
			struct list_head busy_list;	/* fifo list of busy tags */
			int busy;			/* queue depth */
			int max_depth;			/* max queue depth */
		};
	
	Most of the above is simple and straight forward, however busy_list may need
	a bit of explaining. Normally we don't care too much about request ordering,
	but in the event of any barrier requests in the tag queue we need to ensure
	that requests are restarted in the order they were queue. This may happen
	if the driver needs to use blk_queue_invalidate_tags().
	
	3.3 I/O Submission
	
	The routine submit_bio() is used to submit a single io. Higher level i/o
	routines make use of this:
	
	(a) Buffered i/o:
	The routine submit_bh() invokes submit_bio() on a bio corresponding to the
	bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
	
	(b) Kiobuf i/o (for raw/direct i/o):
	The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
	maps the array to one or more multi-page bios, issuing submit_bio() to
	perform the i/o on each of these.
	
	The embedded bh array in the kiobuf structure has been removed and no
	preallocation of bios is done for kiobufs. [The intent is to remove the
	blocks array as well, but it's currently in there to kludge around direct i/o.]
	Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
	
	Todo/Observation:
	
	 A single kiobuf structure is assumed to correspond to a contiguous range
	 of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
	 So right now it wouldn't work for direct i/o on non-contiguous blocks.
	 This is to be resolved.  The eventual direction is to replace kiobuf
	 by kvec's.
	
	 Badari Pulavarty has a patch to implement direct i/o correctly using
	 bio and kvec.
	
	
	(c) Page i/o:
	Todo/Under discussion:
	
	 Andrew Morton's multi-page bio patches attempt to issue multi-page
	 writeouts (and reads) from the page cache, by directly building up
	 large bios for submission completely bypassing the usage of buffer
	 heads. This work is still in progress.
	
	 Christoph Hellwig had some code that uses bios for page-io (rather than
	 bh). This isn't included in bio as yet. Christoph was also working on a
	 design for representing virtual/real extents as an entity and modifying
	 some of the address space ops interfaces to utilize this abstraction rather
	 than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
	 abstraction, but intended to be as lightweight as possible).
	
	(d) Direct access i/o:
	Direct access requests that do not contain bios would be submitted differently
	as discussed earlier in section 1.3.
	
	Aside:
	
	  Kvec i/o:
	
	  Ben LaHaise's aio code uses a slightly different structure instead
	  of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
	  tuples (very much like the networking code), together with a callback function
	  and data pointer. This is embedded into a brw_cb structure when passed
	  to brw_kvec_async().
	
	  Now it should be possible to directly map these kvecs to a bio. Just as while
	  cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
	  array pointer to point to the veclet array in kvecs.
	
	  TBD: In order for this to work, some changes are needed in the way multi-page
	  bios are handled today. The values of the tuples in such a vector passed in
	  from higher level code should not be modified by the block layer in the course
	  of its request processing, since that would make it hard for the higher layer
	  to continue to use the vector descriptor (kvec) after i/o completes. Instead,
	  all such transient state should either be maintained in the request structure,
	  and passed on in some way to the endio completion routine.
	
	
	4. The I/O scheduler
	I/O scheduler, a.k.a. elevator, is implemented in two layers.  Generic dispatch
	queue and specific I/O schedulers.  Unless stated otherwise, elevator is used
	to refer to both parts and I/O scheduler to specific I/O schedulers.
	
	Block layer implements generic dispatch queue in block/*.c.
	The generic dispatch queue is responsible for requeueing, handling non-fs
	requests and all other subtleties.
	
	Specific I/O schedulers are responsible for ordering normal filesystem
	requests.  They can also choose to delay certain requests to improve
	throughput or whatever purpose.  As the plural form indicates, there are
	multiple I/O schedulers.  They can be built as modules but at least one should
	be built inside the kernel.  Each queue can choose different one and can also
	change to another one dynamically.
	
	A block layer call to the i/o scheduler follows the convention elv_xxx(). This
	calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx
	and xxx might not match exactly, but use your imagination. If an elevator
	doesn't implement a function, the switch does nothing or some minimal house
	keeping work.
	
	4.1. I/O scheduler API
	
	The functions an elevator may implement are: (* are mandatory)
	elevator_merge_fn		called to query requests for merge with a bio
	
	elevator_merge_req_fn		called when two requests get merged. the one
					which gets merged into the other one will be
					never seen by I/O scheduler again. IOW, after
					being merged, the request is gone.
	
	elevator_merged_fn		called when a request in the scheduler has been
					involved in a merge. It is used in the deadline
					scheduler for example, to reposition the request
					if its sorting order has changed.
	
	elevator_allow_merge_fn		called whenever the block layer determines
					that a bio can be merged into an existing
					request safely. The io scheduler may still
					want to stop a merge at this point if it
					results in some sort of conflict internally,
					this hook allows it to do that. Note however
					that two *requests* can still be merged at later
					time. Currently the io scheduler has no way to
					prevent that. It can only learn about the fact
					from elevator_merge_req_fn callback.
	
	elevator_dispatch_fn*		fills the dispatch queue with ready requests.
					I/O schedulers are free to postpone requests by
					not filling the dispatch queue unless @force
					is non-zero.  Once dispatched, I/O schedulers
					are not allowed to manipulate the requests -
					they belong to generic dispatch queue.
	
	elevator_add_req_fn*		called to add a new request into the scheduler
	
	elevator_former_req_fn
	elevator_latter_req_fn		These return the request before or after the
					one specified in disk sort order. Used by the
					block layer to find merge possibilities.
	
	elevator_completed_req_fn	called when a request is completed.
	
	elevator_may_queue_fn		returns true if the scheduler wants to allow the
					current context to queue a new request even if
					it is over the queue limit. This must be used
					very carefully!!
	
	elevator_set_req_fn
	elevator_put_req_fn		Must be used to allocate and free any elevator
					specific storage for a request.
	
	elevator_activate_req_fn	Called when device driver first sees a request.
					I/O schedulers can use this callback to
					determine when actual execution of a request
					starts.
	elevator_deactivate_req_fn	Called when device driver decides to delay
					a request by requeueing it.
	
	elevator_init_fn*
	elevator_exit_fn		Allocate and free any elevator specific storage
					for a queue.
	
	4.2 Request flows seen by I/O schedulers
	All requests seen by I/O schedulers strictly follow one of the following three
	flows.
	
	 set_req_fn ->
	
	 i.   add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
	      (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
	 ii.  add_req_fn -> (merged_fn ->)* -> merge_req_fn
	 iii. [none]
	
	 -> put_req_fn
	
	4.3 I/O scheduler implementation
	The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
	optimal disk scan and request servicing performance (based on generic
	principles and device capabilities), optimized for:
	i.   improved throughput
	ii.  improved latency
	iii. better utilization of h/w & CPU time
	
	Characteristics:
	
	i. Binary tree
	AS and deadline i/o schedulers use red black binary trees for disk position
	sorting and searching, and a fifo linked list for time-based searching. This
	gives good scalability and good availability of information. Requests are
	almost always dispatched in disk sort order, so a cache is kept of the next
	request in sort order to prevent binary tree lookups.
	
	This arrangement is not a generic block layer characteristic however, so
	elevators may implement queues as they please.
	
	ii. Merge hash
	AS and deadline use a hash table indexed by the last sector of a request. This
	enables merging code to quickly look up "back merge" candidates, even when
	multiple I/O streams are being performed at once on one disk.
	
	"Front merges", a new request being merged at the front of an existing request,
	are far less common than "back merges" due to the nature of most I/O patterns.
	Front merges are handled by the binary trees in AS and deadline schedulers.
	
	iii. Plugging the queue to batch requests in anticipation of opportunities for
	     merge/sort optimizations
	
	Plugging is an approach that the current i/o scheduling algorithm resorts to so
	that it collects up enough requests in the queue to be able to take
	advantage of the sorting/merging logic in the elevator. If the
	queue is empty when a request comes in, then it plugs the request queue
	(sort of like plugging the bath tub of a vessel to get fluid to build up)
	till it fills up with a few more requests, before starting to service
	the requests. This provides an opportunity to merge/sort the requests before
	passing them down to the device. There are various conditions when the queue is
	unplugged (to open up the flow again), either through a scheduled task or
	could be on demand. For example wait_on_buffer sets the unplugging going
	through sync_buffer() running blk_run_address_space(mapping). Or the caller
	can do it explicity through blk_unplug(bdev). So in the read case,
	the queue gets explicitly unplugged as part of waiting for completion on that
	buffer.
	
	Aside:
	  This is kind of controversial territory, as it's not clear if plugging is
	  always the right thing to do. Devices typically have their own queues,
	  and allowing a big queue to build up in software, while letting the device be
	  idle for a while may not always make sense. The trick is to handle the fine
	  balance between when to plug and when to open up. Also now that we have
	  multi-page bios being queued in one shot, we may not need to wait to merge
	  a big request from the broken up pieces coming by.
	
	4.4 I/O contexts
	I/O contexts provide a dynamically allocated per process data area. They may
	be used in I/O schedulers, and in the block layer (could be used for IO statis,
	priorities for example). See *io_context in block/ll_rw_blk.c, and as-iosched.c
	for an example of usage in an i/o scheduler.
	
	
	5. Scalability related changes
	
	5.1 Granular Locking: io_request_lock replaced by a per-queue lock
	
	The global io_request_lock has been removed as of 2.5, to avoid
	the scalability bottleneck it was causing, and has been replaced by more
	granular locking. The request queue structure has a pointer to the
	lock to be used for that queue. As a result, locking can now be
	per-queue, with a provision for sharing a lock across queues if
	necessary (e.g the scsi layer sets the queue lock pointers to the
	corresponding adapter lock, which results in a per host locking
	granularity). The locking semantics are the same, i.e. locking is
	still imposed by the block layer, grabbing the lock before
	request_fn execution which it means that lots of older drivers
	should still be SMP safe. Drivers are free to drop the queue
	lock themselves, if required. Drivers that explicitly used the
	io_request_lock for serialization need to be modified accordingly.
	Usually it's as easy as adding a global lock:
	
		static DEFINE_SPINLOCK(my_driver_lock);
	
	and passing the address to that lock to blk_init_queue().
	
	5.2 64 bit sector numbers (sector_t prepares for 64 bit support)
	
	The sector number used in the bio structure has been changed to sector_t,
	which could be defined as 64 bit in preparation for 64 bit sector support.
	
	6. Other Changes/Implications
	
	6.1 Partition re-mapping handled by the generic block layer
	
	In 2.5 some of the gendisk/partition related code has been reorganized.
	Now the generic block layer performs partition-remapping early and thus
	provides drivers with a sector number relative to whole device, rather than
	having to take partition number into account in order to arrive at the true
	sector number. The routine blk_partition_remap() is invoked by
	generic_make_request even before invoking the queue specific make_request_fn,
	so the i/o scheduler also gets to operate on whole disk sector numbers. This
	should typically not require changes to block drivers, it just never gets
	to invoke its own partition sector offset calculations since all bios
	sent are offset from the beginning of the device.
	
	
	7. A Few Tips on Migration of older drivers
	
	Old-style drivers that just use CURRENT and ignores clustered requests,
	may not need much change.  The generic layer will automatically handle
	clustered requests, multi-page bios, etc for the driver.
	
	For a low performance driver or hardware that is PIO driven or just doesn't
	support scatter-gather changes should be minimal too.
	
	The following are some points to keep in mind when converting old drivers
	to bio.
	
	Drivers should use elv_next_request to pick up requests and are no longer
	supposed to handle looping directly over the request list.
	(struct request->queue has been removed)
	
	Now end_that_request_first takes an additional number_of_sectors argument.
	It used to handle always just the first buffer_head in a request, now
	it will loop and handle as many sectors (on a bio-segment granularity)
	as specified.
	
	Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
	right thing to use is bio_endio(bio) instead.
	
	If the driver is dropping the io_request_lock from its request_fn strategy,
	then it just needs to replace that with q->queue_lock instead.
	
	As described in Sec 1.1, drivers can set max sector size, max segment size
	etc per queue now. Drivers that used to define their own merge functions i
	to handle things like this can now just use the blk_queue_* functions at
	blk_init_queue time.
	
	Drivers no longer have to map a {partition, sector offset} into the
	correct absolute location anymore, this is done by the block layer, so
	where a driver received a request ala this before:
	
		rq->rq_dev = mk_kdev(3, 5);	/* /dev/hda5 */
		rq->sector = 0;			/* first sector on hda5 */
	
	  it will now see
	
		rq->rq_dev = mk_kdev(3, 0);	/* /dev/hda */
		rq->sector = 123128;		/* offset from start of disk */
	
	As mentioned, there is no virtual mapping of a bio. For DMA, this is
	not a problem as the driver probably never will need a virtual mapping.
	Instead it needs a bus mapping (dma_map_page for a single segment or
	use dma_map_sg for scatter gather) to be able to ship it to the driver. For
	PIO drivers (or drivers that need to revert to PIO transfer once in a
	while (IDE for example)), where the CPU is doing the actual data
	transfer a virtual mapping is needed. If the driver supports highmem I/O,
	(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map
	a bio into the virtual address space.
	
	
	8. Prior/Related/Impacted patches
	
	8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
	- orig kiobuf & raw i/o patches (now in 2.4 tree)
	- direct kiobuf based i/o to devices (no intermediate bh's)
	- page i/o using kiobuf
	- kiobuf splitting for lvm (mkp)
	- elevator support for kiobuf request merging (axboe)
	8.2. Zero-copy networking (Dave Miller)
	8.3. SGI XFS - pagebuf patches - use of kiobufs
	8.4. Multi-page pioent patch for bio (Christoph Hellwig)
	8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
	8.6. Async i/o implementation patch (Ben LaHaise)
	8.7. EVMS layering design (IBM EVMS team)
	8.8. Larger page cache size patch (Ben LaHaise) and
	     Large page size (Daniel Phillips)
	    => larger contiguous physical memory buffers
	8.9. VM reservations patch (Ben LaHaise)
	8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
	8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
	8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar,
	      Badari)
	8.13  Priority based i/o scheduler - prepatches (Arjan van de Ven)
	8.14  IDE Taskfile i/o patch (Andre Hedrick)
	8.15  Multi-page writeout and readahead patches (Andrew Morton)
	8.16  Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
	
	9. Other References:
	
	9.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml,
	and Linus' comments - Jan 2001)
	9.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan
	et al - Feb-March 2001 (many of the initial thoughts that led to bio were
	brought up in this discussion thread)
	9.3 Discussions on mempool on lkml - Dec 2001.

• [ block ]
• 00-INDEX
• bfq-iosched.txt
• biodoc.txt
• biovecs.txt
• capability.txt
• cfq-iosched.txt
• cmdline-partition.txt
• data-integrity.txt
• deadline-iosched.txt
• ioprio.txt
• kyber-iosched.txt
• null_blk.txt
• pr.txt
• queue-sysfs.txt
• request.txt
• stat.txt
• switching-sched.txt
• writeback_cache_control.txt
•  

---