1=============================
2Examining Process Page Tables
3=============================
4
5pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
6userspace programs to examine the page tables and related information by
7reading files in ``/proc``.
8
9There are four components to pagemap:
10
11 * ``/proc/pid/pagemap``.  This file lets a userspace process find out which
12   physical frame each virtual page is mapped to.  It contains one 64-bit
13   value for each virtual page, containing the following data (from
14   ``fs/proc/task_mmu.c``, above pagemap_read):
15
16    * Bits 0-54  page frame number (PFN) if present
17    * Bits 0-4   swap type if swapped
18    * Bits 5-54  swap offset if swapped
19    * Bit  55    pte is soft-dirty (see
20      Documentation/admin-guide/mm/soft-dirty.rst)
21    * Bit  56    page exclusively mapped (since 4.2)
22    * Bit  57    pte is uffd-wp write-protected (since 5.13) (see
23      Documentation/admin-guide/mm/userfaultfd.rst)
24    * Bits 58-60 zero
25    * Bit  61    page is file-page or shared-anon (since 3.5)
26    * Bit  62    page swapped
27    * Bit  63    page present
28
29   Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs.
30   In 4.0 and 4.1 opens by unprivileged fail with -EPERM.  Starting from
31   4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN.
32   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.
33
34   If the page is not present but in swap, then the PFN contains an
35   encoding of the swap file number and the page's offset into the
36   swap. Unmapped pages return a null PFN. This allows determining
37   precisely which pages are mapped (or in swap) and comparing mapped
38   pages between processes.
39
40   Efficient users of this interface will use ``/proc/pid/maps`` to
41   determine which areas of memory are actually mapped and llseek to
42   skip over unmapped regions.
43
44 * ``/proc/kpagecount``.  This file contains a 64-bit count of the number of
45   times each page is mapped, indexed by PFN.
46
47The page-types tool in the tools/mm directory can be used to query the
48number of times a page is mapped.
49
50 * ``/proc/kpageflags``.  This file contains a 64-bit set of flags for each
51   page, indexed by PFN.
52
53   The flags are (from ``fs/proc/page.c``, above kpageflags_read):
54
55    0. LOCKED
56    1. ERROR
57    2. REFERENCED
58    3. UPTODATE
59    4. DIRTY
60    5. LRU
61    6. ACTIVE
62    7. SLAB
63    8. WRITEBACK
64    9. RECLAIM
65    10. BUDDY
66    11. MMAP
67    12. ANON
68    13. SWAPCACHE
69    14. SWAPBACKED
70    15. COMPOUND_HEAD
71    16. COMPOUND_TAIL
72    17. HUGE
73    18. UNEVICTABLE
74    19. HWPOISON
75    20. NOPAGE
76    21. KSM
77    22. THP
78    23. OFFLINE
79    24. ZERO_PAGE
80    25. IDLE
81    26. PGTABLE
82
83 * ``/proc/kpagecgroup``.  This file contains a 64-bit inode number of the
84   memory cgroup each page is charged to, indexed by PFN. Only available when
85   CONFIG_MEMCG is set.
86
87Short descriptions to the page flags
88====================================
89
900 - LOCKED
91   The page is being locked for exclusive access, e.g. by undergoing read/write
92   IO.
937 - SLAB
94   The page is managed by the SLAB/SLUB kernel memory allocator.
95   When compound page is used, either will only set this flag on the head
96   page.
9710 - BUDDY
98    A free memory block managed by the buddy system allocator.
99    The buddy system organizes free memory in blocks of various orders.
100    An order N block has 2^N physically contiguous pages, with the BUDDY flag
101    set for and _only_ for the first page.
10215 - COMPOUND_HEAD
103    A compound page with order N consists of 2^N physically contiguous pages.
104    A compound page with order 2 takes the form of "HTTT", where H donates its
105    head page and T donates its tail page(s).  The major consumers of compound
106    pages are hugeTLB pages (Documentation/admin-guide/mm/hugetlbpage.rst),
107    the SLUB etc.  memory allocators and various device drivers.
108    However in this interface, only huge/giga pages are made visible
109    to end users.
11016 - COMPOUND_TAIL
111    A compound page tail (see description above).
11217 - HUGE
113    This is an integral part of a HugeTLB page.
11419 - HWPOISON
115    Hardware detected memory corruption on this page: don't touch the data!
11620 - NOPAGE
117    No page frame exists at the requested address.
11821 - KSM
119    Identical memory pages dynamically shared between one or more processes.
12022 - THP
121    Contiguous pages which construct transparent hugepages.
12223 - OFFLINE
123    The page is logically offline.
12424 - ZERO_PAGE
125    Zero page for pfn_zero or huge_zero page.
12625 - IDLE
127    The page has not been accessed since it was marked idle (see
128    Documentation/admin-guide/mm/idle_page_tracking.rst).
129    Note that this flag may be stale in case the page was accessed via
130    a PTE. To make sure the flag is up-to-date one has to read
131    ``/sys/kernel/mm/page_idle/bitmap`` first.
13226 - PGTABLE
133    The page is in use as a page table.
134
135IO related page flags
136---------------------
137
1381 - ERROR
139   IO error occurred.
1403 - UPTODATE
141   The page has up-to-date data.
142   ie. for file backed page: (in-memory data revision >= on-disk one)
1434 - DIRTY
144   The page has been written to, hence contains new data.
145   i.e. for file backed page: (in-memory data revision >  on-disk one)
1468 - WRITEBACK
147   The page is being synced to disk.
148
149LRU related page flags
150----------------------
151
1525 - LRU
153   The page is in one of the LRU lists.
1546 - ACTIVE
155   The page is in the active LRU list.
15618 - UNEVICTABLE
157   The page is in the unevictable (non-)LRU list It is somehow pinned and
158   not a candidate for LRU page reclaims, e.g. ramfs pages,
159   shmctl(SHM_LOCK) and mlock() memory segments.
1602 - REFERENCED
161   The page has been referenced since last LRU list enqueue/requeue.
1629 - RECLAIM
163   The page will be reclaimed soon after its pageout IO completed.
16411 - MMAP
165   A memory mapped page.
16612 - ANON
167   A memory mapped page that is not part of a file.
16813 - SWAPCACHE
169   The page is mapped to swap space, i.e. has an associated swap entry.
17014 - SWAPBACKED
171   The page is backed by swap/RAM.
172
173The page-types tool in the tools/mm directory can be used to query the
174above flags.
175
176Using pagemap to do something useful
177====================================
178
179The general procedure for using pagemap to find out about a process' memory
180usage goes like this:
181
182 1. Read ``/proc/pid/maps`` to determine which parts of the memory space are
183    mapped to what.
184 2. Select the maps you are interested in -- all of them, or a particular
185    library, or the stack or the heap, etc.
186 3. Open ``/proc/pid/pagemap`` and seek to the pages you would like to examine.
187 4. Read a u64 for each page from pagemap.
188 5. Open ``/proc/kpagecount`` and/or ``/proc/kpageflags``.  For each PFN you
189    just read, seek to that entry in the file, and read the data you want.
190
191For example, to find the "unique set size" (USS), which is the amount of
192memory that a process is using that is not shared with any other process,
193you can go through every map in the process, find the PFNs, look those up
194in kpagecount, and tally up the number of pages that are only referenced
195once.
196
197Exceptions for Shared Memory
198============================
199
200Page table entries for shared pages are cleared when the pages are zapped or
201swapped out. This makes swapped out pages indistinguishable from never-allocated
202ones.
203
204In kernel space, the swap location can still be retrieved from the page cache.
205However, values stored only on the normal PTE get lost irretrievably when the
206page is swapped out (i.e. SOFT_DIRTY).
207
208In user space, whether the page is present, swapped or none can be deduced with
209the help of lseek and/or mincore system calls.
210
211lseek() can differentiate between accessed pages (present or swapped out) and
212holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where
213the pages are backed. For anonymous shared pages, the file can be found in
214``/proc/pid/map_files/``.
215
216mincore() can differentiate between pages in memory (present, including swap
217cache) and out of memory (swapped out or none/non-allocated).
218
219Other notes
220===========
221
222Reading from any of the files will return -EINVAL if you are not starting
223the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
224into the file), or if the size of the read is not a multiple of 8 bytes.
225
226Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
227always 12 at most architectures). Since Linux 3.11 their meaning changes
228after first clear of soft-dirty bits. Since Linux 4.2 they are used for
229flags unconditionally.
230