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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 16. 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.