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 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
79Short 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
10619. HWPOISON
107    hardware detected memory corruption on this page: don't touch the data!
108
10920. NOPAGE
110    no page frame exists at the requested address
111
11221. KSM
113    identical memory pages dynamically shared between one or more processes
114
11522. THP
116    contiguous pages which construct transparent hugepages
117
11823. BALLOON
119    balloon compaction page
120
12124. ZERO_PAGE
122    zero page for pfn_zero or huge_zero page
123
12425. 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
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
151The page-types tool in the tools/vm directory can be used to query the
152above flags.
153
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
177the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
178into the file), or if the size of the read is not a multiple of 8 bytes.
179
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.
184