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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
Vladimir Davydov80ae2fd2015-09-09 15:35:38 -07008There are four components to pagemap:
Thomas Tuttleef421be2008-06-05 22:46:59 -07009
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
Pavel Emelyanov541c2372013-07-03 15:01:22 -070018 * Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
Konstantin Khlebnikov83b4b0b2015-09-08 15:00:13 -070019 * Bit 56 page exclusively mapped (since 4.2)
Konstantin Khlebnikov77bb4992015-09-08 15:00:10 -070020 * Bits 57-60 zero
Konstantin Khlebnikov83b4b0b2015-09-08 15:00:13 -070021 * Bit 61 page is file-page or shared-anon (since 3.5)
Thomas Tuttleef421be2008-06-05 22:46:59 -070022 * Bit 62 page swapped
23 * Bit 63 page present
24
Konstantin Khlebnikov83b4b0b2015-09-08 15:00:13 -070025 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
Thomas Tuttleef421be2008-06-05 22:46:59 -070030 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
Wu Fengguangc9ba78e2009-06-16 15:32:25 -070046 The flags are (from fs/proc/page.c, above kpageflags_read):
Thomas Tuttleef421be2008-06-05 22:46:59 -070047
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
Wu Fengguang17e89502009-06-16 15:32:26 -070059 11. MMAP
60 12. ANON
61 13. SWAPCACHE
62 14. SWAPBACKED
63 15. COMPOUND_HEAD
64 16. COMPOUND_TAIL
Doug Hoyte63f8e8d2016-04-13 11:09:21 -040065 17. HUGE
Wu Fengguang17e89502009-06-16 15:32:26 -070066 18. UNEVICTABLE
Wu Fengguang253fb022009-10-07 16:32:27 -070067 19. HWPOISON
Wu Fengguang17e89502009-06-16 15:32:26 -070068 20. NOPAGE
Wu Fengguanga1bbb5e2009-10-07 16:32:28 -070069 21. KSM
Naoya Horiguchi807f0cc2012-03-21 16:33:58 -070070 22. THP
Wang, Yalin56873f42015-02-11 15:24:51 -080071 23. BALLOON
72 24. ZERO_PAGE
Vladimir Davydovf074a8f2015-09-09 15:35:48 -070073 25. IDLE
Wu Fengguang17e89502009-06-16 15:32:26 -070074
Vladimir Davydov80ae2fd2015-09-09 15:35:38 -070075 * /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
Wu Fengguang17e89502009-06-16 15:32:26 -070079Short 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
8910. 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
9515. COMPOUND_HEAD
9616. 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.
10317. HUGE
104 this is an integral part of a HugeTLB page
105
Wu Fengguang253fb022009-10-07 16:32:27 -070010619. HWPOISON
107 hardware detected memory corruption on this page: don't touch the data!
108
Wu Fengguang17e89502009-06-16 15:32:26 -070010920. NOPAGE
110 no page frame exists at the requested address
111
Wu Fengguanga1bbb5e2009-10-07 16:32:28 -070011221. KSM
113 identical memory pages dynamically shared between one or more processes
114
Naoya Horiguchi807f0cc2012-03-21 16:33:58 -070011522. THP
116 contiguous pages which construct transparent hugepages
117
Wang, Yalin56873f42015-02-11 15:24:51 -080011823. BALLOON
119 balloon compaction page
120
12124. ZERO_PAGE
122 zero page for pfn_zero or huge_zero page
123
Vladimir Davydovf074a8f2015-09-09 15:35:48 -070012425. 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
Wu Fengguang17e89502009-06-16 15:32:26 -0700130 [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
14118. 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
14611. MMAP a memory mapped page
14712. ANON a memory mapped page that is not part of a file
14813. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry
14914. SWAPBACKED page is backed by swap/RAM
150
Randy Wright3250af12015-04-10 15:00:02 -0600151The page-types tool in the tools/vm directory can be used to query the
152above flags.
Thomas Tuttleef421be2008-06-05 22:46:59 -0700153
154Using pagemap to do something useful:
155
156The general procedure for using pagemap to find out about a process' memory
157usage 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
168For example, to find the "unique set size" (USS), which is the amount of
169memory that a process is using that is not shared with any other process,
170you can go through every map in the process, find the PFNs, look those up
171in kpagecount, and tally up the number of pages that are only referenced
172once.
173
174Other notes:
175
176Reading from any of the files will return -EINVAL if you are not starting
Anatol Pomozovf884ab12013-05-08 16:56:16 -0700177the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
Thomas Tuttleef421be2008-06-05 22:46:59 -0700178into the file), or if the size of the read is not a multiple of 8 bytes.
Konstantin Khlebnikov83b4b0b2015-09-08 15:00:13 -0700179
180Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
181always 12 at most architectures). Since Linux 3.11 their meaning changes
182after first clear of soft-dirty bits. Since Linux 4.2 they are used for
183flags unconditionally.