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Thomas Tuttleef421be2008-06-05 22:46:59 -07001pagemap, 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
Wu Fengguangc9ba78e2009-06-16 15:32:25 -070015 * Bits 0-54 page frame number (PFN) if present
Thomas Tuttleef421be2008-06-05 22:46:59 -070016 * Bits 0-4 swap type if swapped
Wu Fengguangc9ba78e2009-06-16 15:32:25 -070017 * Bits 5-54 swap offset if swapped
Thomas Tuttleef421be2008-06-05 22:46:59 -070018 * 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
Wu Fengguangc9ba78e2009-06-16 15:32:25 -070039 The flags are (from fs/proc/page.c, above kpageflags_read):
Thomas Tuttleef421be2008-06-05 22:46:59 -070040
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
Wu Fengguang17e89502009-06-16 15:32:26 -070052 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 20. NOPAGE
61
62Short descriptions to the page flags:
63
64 0. LOCKED
65 page is being locked for exclusive access, eg. by undergoing read/write IO
66
67 7. SLAB
68 page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
69 When compound page is used, SLUB/SLQB will only set this flag on the head
70 page; SLOB will not flag it at all.
71
7210. BUDDY
73 a free memory block managed by the buddy system allocator
74 The buddy system organizes free memory in blocks of various orders.
75 An order N block has 2^N physically contiguous pages, with the BUDDY flag
76 set for and _only_ for the first page.
77
7815. COMPOUND_HEAD
7916. COMPOUND_TAIL
80 A compound page with order N consists of 2^N physically contiguous pages.
81 A compound page with order 2 takes the form of "HTTT", where H donates its
82 head page and T donates its tail page(s). The major consumers of compound
83 pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
84 memory allocators and various device drivers. However in this interface,
85 only huge/giga pages are made visible to end users.
8617. HUGE
87 this is an integral part of a HugeTLB page
88
8920. NOPAGE
90 no page frame exists at the requested address
91
92 [IO related page flags]
93 1. ERROR IO error occurred
94 3. UPTODATE page has up-to-date data
95 ie. for file backed page: (in-memory data revision >= on-disk one)
96 4. DIRTY page has been written to, hence contains new data
97 ie. for file backed page: (in-memory data revision > on-disk one)
98 8. WRITEBACK page is being synced to disk
99
100 [LRU related page flags]
101 5. LRU page is in one of the LRU lists
102 6. ACTIVE page is in the active LRU list
10318. UNEVICTABLE page is in the unevictable (non-)LRU list
104 It is somehow pinned and not a candidate for LRU page reclaims,
105 eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
106 2. REFERENCED page has been referenced since last LRU list enqueue/requeue
107 9. RECLAIM page will be reclaimed soon after its pageout IO completed
10811. MMAP a memory mapped page
10912. ANON a memory mapped page that is not part of a file
11013. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry
11114. SWAPBACKED page is backed by swap/RAM
112
113The page-types tool in this directory can be used to query the above flags.
Thomas Tuttleef421be2008-06-05 22:46:59 -0700114
115Using pagemap to do something useful:
116
117The general procedure for using pagemap to find out about a process' memory
118usage goes like this:
119
120 1. Read /proc/pid/maps to determine which parts of the memory space are
121 mapped to what.
122 2. Select the maps you are interested in -- all of them, or a particular
123 library, or the stack or the heap, etc.
124 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
125 4. Read a u64 for each page from pagemap.
126 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just
127 read, seek to that entry in the file, and read the data you want.
128
129For example, to find the "unique set size" (USS), which is the amount of
130memory that a process is using that is not shared with any other process,
131you can go through every map in the process, find the PFNs, look those up
132in kpagecount, and tally up the number of pages that are only referenced
133once.
134
135Other notes:
136
137Reading from any of the files will return -EINVAL if you are not starting
138the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
139into the file), or if the size of the read is not a multiple of 8 bytes.