Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 pagemap, from the userspace perspective 2 --------------------------------------- 3 4 pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow 5 userspace programs to examine the page tables and related information by 6 reading files in /proc. 7 8 There are four components to pagemap: 9 10 * /proc/pid/pagemap. This file lets a userspace process find out which 11 physical frame each virtual page is mapped to. It contains one 64-bit 12 value for each virtual page, containing the following data (from 13 fs/proc/task_mmu.c, above pagemap_read): 14 15 * Bits 0-54 page frame number (PFN) if present 16 * Bits 0-4 swap type if swapped 17 * Bits 5-54 swap offset if swapped 18 * Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt) 19 * Bit 56 page exclusively mapped (since 4.2) 20 * Bits 57-60 zero 21 * Bit 61 page is file-page or shared-anon (since 3.5) 22 * Bit 62 page swapped 23 * Bit 63 page present 24 25 Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs. 26 In 4.0 and 4.1 opens by unprivileged fail with -EPERM. Starting from 27 4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN. 28 Reason: information about PFNs helps in exploiting Rowhammer vulnerability. 29 30 If the page is not present but in swap, then the PFN contains an 31 encoding of the swap file number and the page's offset into the 32 swap. Unmapped pages return a null PFN. This allows determining 33 precisely which pages are mapped (or in swap) and comparing mapped 34 pages between processes. 35 36 Efficient users of this interface will use /proc/pid/maps to 37 determine which areas of memory are actually mapped and llseek to 38 skip over unmapped regions. 39 40 * /proc/kpagecount. This file contains a 64-bit count of the number of 41 times each page is mapped, indexed by PFN. 42 43 * /proc/kpageflags. This file contains a 64-bit set of flags for each 44 page, indexed by PFN. 45 46 The flags are (from fs/proc/page.c, above kpageflags_read): 47 48 0. LOCKED 49 1. ERROR 50 2. REFERENCED 51 3. UPTODATE 52 4. DIRTY 53 5. LRU 54 6. ACTIVE 55 7. SLAB 56 8. WRITEBACK 57 9. RECLAIM 58 10. BUDDY 59 11. MMAP 60 12. ANON 61 13. SWAPCACHE 62 14. SWAPBACKED 63 15. COMPOUND_HEAD 64 16. COMPOUND_TAIL 65 17. HUGE 66 18. UNEVICTABLE 67 19. HWPOISON 68 20. NOPAGE 69 21. KSM 70 22. THP 71 23. BALLOON 72 24. ZERO_PAGE 73 25. IDLE 74 75 * /proc/kpagecgroup. This file contains a 64-bit inode number of the 76 memory cgroup each page is charged to, indexed by PFN. Only available when 77 CONFIG_MEMCG is set. 78 79 Short descriptions to the page flags: 80 81 0. LOCKED 82 page is being locked for exclusive access, eg. by undergoing read/write IO 83 84 7. SLAB 85 page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator 86 When compound page is used, SLUB/SLQB will only set this flag on the head 87 page; SLOB will not flag it at all. 88 89 10. BUDDY 90 a free memory block managed by the buddy system allocator 91 The buddy system organizes free memory in blocks of various orders. 92 An order N block has 2^N physically contiguous pages, with the BUDDY flag 93 set for and _only_ for the first page. 94 95 15. COMPOUND_HEAD 96 16. COMPOUND_TAIL 97 A compound page with order N consists of 2^N physically contiguous pages. 98 A compound page with order 2 takes the form of "HTTT", where H donates its 99 head page and T donates its tail page(s). The major consumers of compound 100 pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc. 101 memory allocators and various device drivers. However in this interface, 102 only huge/giga pages are made visible to end users. 103 17. HUGE 104 this is an integral part of a HugeTLB page 105 106 19. HWPOISON 107 hardware detected memory corruption on this page: don't touch the data! 108 109 20. NOPAGE 110 no page frame exists at the requested address 111 112 21. KSM 113 identical memory pages dynamically shared between one or more processes 114 115 22. THP 116 contiguous pages which construct transparent hugepages 117 118 23. BALLOON 119 balloon compaction page 120 121 24. ZERO_PAGE 122 zero page for pfn_zero or huge_zero page 123 124 25. IDLE 125 page has not been accessed since it was marked idle (see 126 Documentation/vm/idle_page_tracking.txt). Note that this flag may be 127 stale in case the page was accessed via a PTE. To make sure the flag 128 is up-to-date one has to read /sys/kernel/mm/page_idle/bitmap first. 129 130 [IO related page flags] 131 1. ERROR IO error occurred 132 3. UPTODATE page has up-to-date data 133 ie. for file backed page: (in-memory data revision >= on-disk one) 134 4. DIRTY page has been written to, hence contains new data 135 ie. for file backed page: (in-memory data revision > on-disk one) 136 8. WRITEBACK page is being synced to disk 137 138 [LRU related page flags] 139 5. LRU page is in one of the LRU lists 140 6. ACTIVE page is in the active LRU list 141 18. UNEVICTABLE page is in the unevictable (non-)LRU list 142 It is somehow pinned and not a candidate for LRU page reclaims, 143 eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments 144 2. REFERENCED page has been referenced since last LRU list enqueue/requeue 145 9. RECLAIM page will be reclaimed soon after its pageout IO completed 146 11. MMAP a memory mapped page 147 12. ANON a memory mapped page that is not part of a file 148 13. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry 149 14. SWAPBACKED page is backed by swap/RAM 150 151 The page-types tool in the tools/vm directory can be used to query the 152 above flags. 153 154 Using pagemap to do something useful: 155 156 The general procedure for using pagemap to find out about a process' memory 157 usage goes like this: 158 159 1. Read /proc/pid/maps to determine which parts of the memory space are 160 mapped to what. 161 2. Select the maps you are interested in -- all of them, or a particular 162 library, or the stack or the heap, etc. 163 3. Open /proc/pid/pagemap and seek to the pages you would like to examine. 164 4. Read a u64 for each page from pagemap. 165 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just 166 read, seek to that entry in the file, and read the data you want. 167 168 For example, to find the "unique set size" (USS), which is the amount of 169 memory that a process is using that is not shared with any other process, 170 you can go through every map in the process, find the PFNs, look those up 171 in kpagecount, and tally up the number of pages that are only referenced 172 once. 173 174 Other notes: 175 176 Reading from any of the files will return -EINVAL if you are not starting 177 the read on an 8-byte boundary (e.g., if you sought an odd number of bytes 178 into the file), or if the size of the read is not a multiple of 8 bytes. 179 180 Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is 181 always 12 at most architectures). Since Linux 3.11 their meaning changes 182 after first clear of soft-dirty bits. Since Linux 4.2 they are used for 183 flags unconditionally.