Documentation/vm/pagemap.txt

   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 three 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     * Bits 56-60 zero
  20     * Bit  61    page is file-page or shared-anon
  21     * Bit  62    page swapped
  22     * Bit  63    page present
  23
  24    If the page is not present but in swap, then the PFN contains an
  25    encoding of the swap file number and the page's offset into the
  26    swap. Unmapped pages return a null PFN. This allows determining
  27    precisely which pages are mapped (or in swap) and comparing mapped
  28    pages between processes.
  29
  30    Efficient users of this interface will use /proc/pid/maps to
  31    determine which areas of memory are actually mapped and llseek to
  32    skip over unmapped regions.
  33
  34  * /proc/kpagecount.  This file contains a 64-bit count of the number of
  35    times each page is mapped, indexed by PFN.
  36
  37  * /proc/kpageflags.  This file contains a 64-bit set of flags for each
  38    page, indexed by PFN.
  39
  40    The flags are (from fs/proc/page.c, above kpageflags_read):
  41
  42      0. LOCKED
  43      1. ERROR
  44      2. REFERENCED
  45      3. UPTODATE
  46      4. DIRTY
  47      5. LRU
  48      6. ACTIVE
  49      7. SLAB
  50      8. WRITEBACK
  51      9. RECLAIM
  52     10. BUDDY
  53     11. MMAP
  54     12. ANON
  55     13. SWAPCACHE
  56     14. SWAPBACKED
  57     15. COMPOUND_HEAD
  58     16. COMPOUND_TAIL
  59     16. HUGE
  60     18. UNEVICTABLE
  61     19. HWPOISON
  62     20. NOPAGE
  63     21. KSM
  64     22. THP
  65     23. BALLOON
  66     24. ZERO_PAGE
  67
  68 Short descriptions to the page flags:
  69
  70  0. LOCKED
  71     page is being locked for exclusive access, eg. by undergoing read/write IO
  72
  73  7. SLAB
  74     page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
  75     When compound page is used, SLUB/SLQB will only set this flag on the head
  76     page; SLOB will not flag it at all.
  77
  78 10. BUDDY
  79     a free memory block managed by the buddy system allocator
  80     The buddy system organizes free memory in blocks of various orders.
  81     An order N block has 2^N physically contiguous pages, with the BUDDY flag
  82     set for and _only_ for the first page.
  83
  84 15. COMPOUND_HEAD
  85 16. COMPOUND_TAIL
  86     A compound page with order N consists of 2^N physically contiguous pages.
  87     A compound page with order 2 takes the form of "HTTT", where H donates its
  88     head page and T donates its tail page(s).  The major consumers of compound
  89     pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
  90     memory allocators and various device drivers. However in this interface,
  91     only huge/giga pages are made visible to end users.
  92 17. HUGE
  93     this is an integral part of a HugeTLB page
  94
  95 19. HWPOISON
  96     hardware detected memory corruption on this page: don't touch the data!
  97
  98 20. NOPAGE
  99     no page frame exists at the requested address
 100
 101 21. KSM
 102     identical memory pages dynamically shared between one or more processes
 103
 104 22. THP
 105     contiguous pages which construct transparent hugepages
 106
 107 23. BALLOON
 108     balloon compaction page
 109
 110 24. ZERO_PAGE
 111     zero page for pfn_zero or huge_zero page
 112
 113     [IO related page flags]
 114  1. ERROR     IO error occurred
 115  3. UPTODATE  page has up-to-date data
 116               ie. for file backed page: (in-memory data revision >= on-disk one)
 117  4. DIRTY     page has been written to, hence contains new data
 118               ie. for file backed page: (in-memory data revision >  on-disk one)
 119  8. WRITEBACK page is being synced to disk
 120
 121     [LRU related page flags]
 122  5. LRU         page is in one of the LRU lists
 123  6. ACTIVE      page is in the active LRU list
 124 18. UNEVICTABLE page is in the unevictable (non-)LRU list
 125                 It is somehow pinned and not a candidate for LRU page reclaims,
 126                 eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
 127  2. REFERENCED  page has been referenced since last LRU list enqueue/requeue
 128  9. RECLAIM     page will be reclaimed soon after its pageout IO completed
 129 11. MMAP        a memory mapped page
 130 12. ANON        a memory mapped page that is not part of a file
 131 13. SWAPCACHE   page is mapped to swap space, ie. has an associated swap entry
 132 14. SWAPBACKED  page is backed by swap/RAM
 133
 134 The page-types tool in the tools/vm directory can be used to query the
 135 above flags.
 136
 137 Using pagemap to do something useful:
 138
 139 The general procedure for using pagemap to find out about a process' memory
 140 usage goes like this:
 141
 142  1. Read /proc/pid/maps to determine which parts of the memory space are
 143     mapped to what.
 144  2. Select the maps you are interested in -- all of them, or a particular
 145     library, or the stack or the heap, etc.
 146  3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
 147  4. Read a u64 for each page from pagemap.
 148  5. Open /proc/kpagecount and/or /proc/kpageflags.  For each PFN you just
 149     read, seek to that entry in the file, and read the data you want.
 150
 151 For example, to find the "unique set size" (USS), which is the amount of
 152 memory that a process is using that is not shared with any other process,
 153 you can go through every map in the process, find the PFNs, look those up
 154 in kpagecount, and tally up the number of pages that are only referenced
 155 once.
 156
 157 Other notes:
 158
 159 Reading from any of the files will return -EINVAL if you are not starting
 160 the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
 161 into the file), or if the size of the read is not a multiple of 8 bytes.