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Documentation / vm / page_owner.txt


Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	page owner: Tracking about who allocated each page
2	-----------------------------------------------------------
3	
4	* Introduction
5	
6	page owner is for the tracking about who allocated each page.
7	It can be used to debug memory leak or to find a memory hogger.
8	When allocation happens, information about allocation such as call stack
9	and order of pages is stored into certain storage for each page.
10	When we need to know about status of all pages, we can get and analyze
11	this information.
12	
13	Although we already have tracepoint for tracing page allocation/free,
14	using it for analyzing who allocate each page is rather complex. We need
15	to enlarge the trace buffer for preventing overlapping until userspace
16	program launched. And, launched program continually dump out the trace
17	buffer for later analysis and it would change system behviour with more
18	possibility rather than just keeping it in memory, so bad for debugging.
19	
20	page owner can also be used for various purposes. For example, accurate
21	fragmentation statistics can be obtained through gfp flag information of
22	each page. It is already implemented and activated if page owner is
23	enabled. Other usages are more than welcome.
24	
25	page owner is disabled in default. So, if you'd like to use it, you need
26	to add "page_owner=on" into your boot cmdline. If the kernel is built
27	with page owner and page owner is disabled in runtime due to no enabling
28	boot option, runtime overhead is marginal. If disabled in runtime, it
29	doesn't require memory to store owner information, so there is no runtime
30	memory overhead. And, page owner inserts just two unlikely branches into
31	the page allocator hotpath and if not enabled, then allocation is done
32	like as the kernel without page owner. These two unlikely branches should
33	not affect to allocation performance, especially if the static keys jump
34	label patching functionality is available. Following is the kernel's code
35	size change due to this facility.
36	
37	- Without page owner
38	   text    data     bss     dec     hex filename
39	  40662    1493     644   42799    a72f mm/page_alloc.o
40	
41	- With page owner
42	   text    data     bss     dec     hex filename
43	  40892    1493     644   43029    a815 mm/page_alloc.o
44	   1427      24       8    1459     5b3 mm/page_ext.o
45	   2722      50       0    2772     ad4 mm/page_owner.o
46	
47	Although, roughly, 4 KB code is added in total, page_alloc.o increase by
48	230 bytes and only half of it is in hotpath. Building the kernel with
49	page owner and turning it on if needed would be great option to debug
50	kernel memory problem.
51	
52	There is one notice that is caused by implementation detail. page owner
53	stores information into the memory from struct page extension. This memory
54	is initialized some time later than that page allocator starts in sparse
55	memory system, so, until initialization, many pages can be allocated and
56	they would have no owner information. To fix it up, these early allocated
57	pages are investigated and marked as allocated in initialization phase.
58	Although it doesn't mean that they have the right owner information,
59	at least, we can tell whether the page is allocated or not,
60	more accurately. On 2GB memory x86-64 VM box, 13343 early allocated pages
61	are catched and marked, although they are mostly allocated from struct
62	page extension feature. Anyway, after that, no page is left in
63	un-tracking state.
64	
65	* Usage
66	
67	1) Build user-space helper
68		cd tools/vm
69		make page_owner_sort
70	
71	2) Enable page owner
72		Add "page_owner=on" to boot cmdline.
73	
74	3) Do the job what you want to debug
75	
76	4) Analyze information from page owner
77		cat /sys/kernel/debug/page_owner > page_owner_full.txt
78		grep -v ^PFN page_owner_full.txt > page_owner.txt
79		./page_owner_sort page_owner.txt sorted_page_owner.txt
80	
81		See the result about who allocated each page
82		in the sorted_page_owner.txt.
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