1.. _pagemap:
2
3=============================
4Examining Process Page Tables
5=============================
6
7pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
8userspace programs to examine the page tables and related information by
9reading files in ``/proc``.
10
11There are four components to pagemap:
12
13 * ``/proc/pid/pagemap``.  This file lets a userspace process find out which
14   physical frame each virtual page is mapped to.  It contains one 64-bit
15   value for each virtual page, containing the following data (from
16   ``fs/proc/task_mmu.c``, above pagemap_read):
17
18    * Bits 0-54  page frame number (PFN) if present
19    * Bits 0-4   swap type if swapped
20    * Bits 5-54  swap offset if swapped
21    * Bit  55    pte is soft-dirty (see
22      :ref:`Documentation/admin-guide/mm/soft-dirty.rst <soft_dirty>`)
23    * Bit  56    page exclusively mapped (since 4.2)
24    * Bit  57    pte is uffd-wp write-protected (since 5.13) (see
25      :ref:`Documentation/admin-guide/mm/userfaultfd.rst <userfaultfd>`)
26    * Bits 57-60 zero
27    * Bit  61    page is file-page or shared-anon (since 3.5)
28    * Bit  62    page swapped
29    * Bit  63    page present
30
31   Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs.
32   In 4.0 and 4.1 opens by unprivileged fail with -EPERM.  Starting from
33   4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN.
34   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.
35
36   If the page is not present but in swap, then the PFN contains an
37   encoding of the swap file number and the page's offset into the
38   swap. Unmapped pages return a null PFN. This allows determining
39   precisely which pages are mapped (or in swap) and comparing mapped
40   pages between processes.
41
42   Efficient users of this interface will use ``/proc/pid/maps`` to
43   determine which areas of memory are actually mapped and llseek to
44   skip over unmapped regions.
45
46 * ``/proc/kpagecount``.  This file contains a 64-bit count of the number of
47   times each page is mapped, indexed by PFN.
48
49The page-types tool in the tools/vm directory can be used to query the
50number of times a page is mapped.
51
52 * ``/proc/kpageflags``.  This file contains a 64-bit set of flags for each
53   page, indexed by PFN.
54
55   The flags are (from ``fs/proc/page.c``, above kpageflags_read):
56
57    0. LOCKED
58    1. ERROR
59    2. REFERENCED
60    3. UPTODATE
61    4. DIRTY
62    5. LRU
63    6. ACTIVE
64    7. SLAB
65    8. WRITEBACK
66    9. RECLAIM
67    10. BUDDY
68    11. MMAP
69    12. ANON
70    13. SWAPCACHE
71    14. SWAPBACKED
72    15. COMPOUND_HEAD
73    16. COMPOUND_TAIL
74    17. HUGE
75    18. UNEVICTABLE
76    19. HWPOISON
77    20. NOPAGE
78    21. KSM
79    22. THP
80    23. OFFLINE
81    24. ZERO_PAGE
82    25. IDLE
83    26. PGTABLE
84
85 * ``/proc/kpagecgroup``.  This file contains a 64-bit inode number of the
86   memory cgroup each page is charged to, indexed by PFN. Only available when
87   CONFIG_MEMCG is set.
88
89Short descriptions to the page flags
90====================================
91
920 - LOCKED
93   page is being locked for exclusive access, e.g. by undergoing read/write IO
947 - SLAB
95   page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
96   When compound page is used, SLUB/SLQB will only set this flag on the head
97   page; SLOB will not flag it at all.
9810 - BUDDY
99    a free memory block managed by the buddy system allocator
100    The buddy system organizes free memory in blocks of various orders.
101    An order N block has 2^N physically contiguous pages, with the BUDDY flag
102    set for and _only_ for the first page.
10315 - COMPOUND_HEAD
104    A compound page with order N consists of 2^N physically contiguous pages.
105    A compound page with order 2 takes the form of "HTTT", where H donates its
106    head page and T donates its tail page(s).  The major consumers of compound
107    pages are hugeTLB pages
108    (:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`),
109    the SLUB etc.  memory allocators and various device drivers.
110    However in this interface, only huge/giga pages are made visible
111    to end users.
11216 - COMPOUND_TAIL
113    A compound page tail (see description above).
11417 - HUGE
115    this is an integral part of a HugeTLB page
11619 - HWPOISON
117    hardware detected memory corruption on this page: don't touch the data!
11820 - NOPAGE
119    no page frame exists at the requested address
12021 - KSM
121    identical memory pages dynamically shared between one or more processes
12222 - THP
123    contiguous pages which construct transparent hugepages
12423 - OFFLINE
125    page is logically offline
12624 - ZERO_PAGE
127    zero page for pfn_zero or huge_zero page
12825 - IDLE
129    page has not been accessed since it was marked idle (see
130    :ref:`Documentation/admin-guide/mm/idle_page_tracking.rst <idle_page_tracking>`).
131    Note that this flag may be stale in case the page was accessed via
132    a PTE. To make sure the flag is up-to-date one has to read
133    ``/sys/kernel/mm/page_idle/bitmap`` first.
13426 - PGTABLE
135    page is in use as a page table
136
137IO related page flags
138---------------------
139
1401 - ERROR
141   IO error occurred
1423 - UPTODATE
143   page has up-to-date data
144   ie. for file backed page: (in-memory data revision >= on-disk one)
1454 - DIRTY
146   page has been written to, hence contains new data
147   i.e. for file backed page: (in-memory data revision >  on-disk one)
1488 - WRITEBACK
149   page is being synced to disk
150
151LRU related page flags
152----------------------
153
1545 - LRU
155   page is in one of the LRU lists
1566 - ACTIVE
157   page is in the active LRU list
15818 - UNEVICTABLE
159   page is in the unevictable (non-)LRU list It is somehow pinned and
160   not a candidate for LRU page reclaims, e.g. ramfs pages,
161   shmctl(SHM_LOCK) and mlock() memory segments
1622 - REFERENCED
163   page has been referenced since last LRU list enqueue/requeue
1649 - RECLAIM
165   page will be reclaimed soon after its pageout IO completed
16611 - MMAP
167   a memory mapped page
16812 - ANON
169   a memory mapped page that is not part of a file
17013 - SWAPCACHE
171   page is mapped to swap space, i.e. has an associated swap entry
17214 - SWAPBACKED
173   page is backed by swap/RAM
174
175The page-types tool in the tools/vm directory can be used to query the
176above flags.
177
178Using pagemap to do something useful
179====================================
180
181The general procedure for using pagemap to find out about a process' memory
182usage goes like this:
183
184 1. Read ``/proc/pid/maps`` to determine which parts of the memory space are
185    mapped to what.
186 2. Select the maps you are interested in -- all of them, or a particular
187    library, or the stack or the heap, etc.
188 3. Open ``/proc/pid/pagemap`` and seek to the pages you would like to examine.
189 4. Read a u64 for each page from pagemap.
190 5. Open ``/proc/kpagecount`` and/or ``/proc/kpageflags``.  For each PFN you
191    just read, seek to that entry in the file, and read the data you want.
192
193For example, to find the "unique set size" (USS), which is the amount of
194memory that a process is using that is not shared with any other process,
195you can go through every map in the process, find the PFNs, look those up
196in kpagecount, and tally up the number of pages that are only referenced
197once.
198
199Other notes
200===========
201
202Reading from any of the files will return -EINVAL if you are not starting
203the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
204into the file), or if the size of the read is not a multiple of 8 bytes.
205
206Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
207always 12 at most architectures). Since Linux 3.11 their meaning changes
208after first clear of soft-dirty bits. Since Linux 4.2 they are used for
209flags unconditionally.
210