Root/Documentation/vm/pagemap.txt

1pagemap, from the userspace perspective
2---------------------------------------
3
4pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
5userspace programs to examine the page tables and related information by
6reading files in /proc.
7
8There 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    * Bits 55-60 page shift (page size = 1<<page shift)
19    * Bit 61 reserved for future use
20    * Bit 62 page swapped
21    * Bit 63 page present
22
23   If the page is not present but in swap, then the PFN contains an
24   encoding of the swap file number and the page's offset into the
25   swap. Unmapped pages return a null PFN. This allows determining
26   precisely which pages are mapped (or in swap) and comparing mapped
27   pages between processes.
28
29   Efficient users of this interface will use /proc/pid/maps to
30   determine which areas of memory are actually mapped and llseek to
31   skip over unmapped regions.
32
33 * /proc/kpagecount. This file contains a 64-bit count of the number of
34   times each page is mapped, indexed by PFN.
35
36 * /proc/kpageflags. This file contains a 64-bit set of flags for each
37   page, indexed by PFN.
38
39   The flags are (from fs/proc/page.c, above kpageflags_read):
40
41     0. LOCKED
42     1. ERROR
43     2. REFERENCED
44     3. UPTODATE
45     4. DIRTY
46     5. LRU
47     6. ACTIVE
48     7. SLAB
49     8. WRITEBACK
50     9. RECLAIM
51    10. BUDDY
52    11. MMAP
53    12. ANON
54    13. SWAPCACHE
55    14. SWAPBACKED
56    15. COMPOUND_HEAD
57    16. COMPOUND_TAIL
58    16. HUGE
59    18. UNEVICTABLE
60    19. HWPOISON
61    20. NOPAGE
62    21. KSM
63
64Short descriptions to the page flags:
65
66 0. LOCKED
67    page is being locked for exclusive access, eg. by undergoing read/write IO
68
69 7. SLAB
70    page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
71    When compound page is used, SLUB/SLQB will only set this flag on the head
72    page; SLOB will not flag it at all.
73
7410. BUDDY
75    a free memory block managed by the buddy system allocator
76    The buddy system organizes free memory in blocks of various orders.
77    An order N block has 2^N physically contiguous pages, with the BUDDY flag
78    set for and _only_ for the first page.
79
8015. COMPOUND_HEAD
8116. COMPOUND_TAIL
82    A compound page with order N consists of 2^N physically contiguous pages.
83    A compound page with order 2 takes the form of "HTTT", where H donates its
84    head page and T donates its tail page(s). The major consumers of compound
85    pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
86    memory allocators and various device drivers. However in this interface,
87    only huge/giga pages are made visible to end users.
8817. HUGE
89    this is an integral part of a HugeTLB page
90
9119. HWPOISON
92    hardware detected memory corruption on this page: don't touch the data!
93
9420. NOPAGE
95    no page frame exists at the requested address
96
9721. KSM
98    identical memory pages dynamically shared between one or more processes
99
100    [IO related page flags]
101 1. ERROR IO error occurred
102 3. UPTODATE page has up-to-date data
103              ie. for file backed page: (in-memory data revision >= on-disk one)
104 4. DIRTY page has been written to, hence contains new data
105              ie. for file backed page: (in-memory data revision > on-disk one)
106 8. WRITEBACK page is being synced to disk
107
108    [LRU related page flags]
109 5. LRU page is in one of the LRU lists
110 6. ACTIVE page is in the active LRU list
11118. UNEVICTABLE page is in the unevictable (non-)LRU list
112                It is somehow pinned and not a candidate for LRU page reclaims,
113        eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
114 2. REFERENCED page has been referenced since last LRU list enqueue/requeue
115 9. RECLAIM page will be reclaimed soon after its pageout IO completed
11611. MMAP a memory mapped page
11712. ANON a memory mapped page that is not part of a file
11813. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry
11914. SWAPBACKED page is backed by swap/RAM
120
121The page-types tool in this directory can be used to query the above flags.
122
123Using pagemap to do something useful:
124
125The general procedure for using pagemap to find out about a process' memory
126usage goes like this:
127
128 1. Read /proc/pid/maps to determine which parts of the memory space are
129    mapped to what.
130 2. Select the maps you are interested in -- all of them, or a particular
131    library, or the stack or the heap, etc.
132 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
133 4. Read a u64 for each page from pagemap.
134 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just
135    read, seek to that entry in the file, and read the data you want.
136
137For example, to find the "unique set size" (USS), which is the amount of
138memory that a process is using that is not shared with any other process,
139you can go through every map in the process, find the PFNs, look those up
140in kpagecount, and tally up the number of pages that are only referenced
141once.
142
143Other notes:
144
145Reading from any of the files will return -EINVAL if you are not starting
146the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
147into the file), or if the size of the read is not a multiple of 8 bytes.
148

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