3 =============================
4 Examining Process Page Tables
5 =============================
7 pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
8 userspace programs to examine the page tables and related information by
9 reading files in ``/proc``.
11 There are four components to pagemap:
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):
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)
25 * Bit 61 page is file-page or shared-anon (since 3.5)
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.
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.
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.
44 * ``/proc/kpagecount``. This file contains a 64-bit count of the number of
45 times each page is mapped, indexed by PFN.
47 * ``/proc/kpageflags``. This file contains a 64-bit set of flags for each
50 The flags are (from ``fs/proc/page.c``, above kpageflags_read):
79 * ``/proc/kpagecgroup``. This file contains a 64-bit inode number of the
80 memory cgroup each page is charged to, indexed by PFN. Only available when
83 Short descriptions to the page flags
84 ====================================
87 page is being locked for exclusive access, e.g. by undergoing read/write IO
89 page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
90 When compound page is used, SLUB/SLQB will only set this flag on the head
91 page; SLOB will not flag it at all.
93 a free memory block managed by the buddy system allocator
94 The buddy system organizes free memory in blocks of various orders.
95 An order N block has 2^N physically contiguous pages, with the BUDDY flag
96 set for and _only_ for the first page.
98 A compound page with order N consists of 2^N physically contiguous pages.
99 A compound page with order 2 takes the form of "HTTT", where H donates its
100 head page and T donates its tail page(s). The major consumers of compound
101 pages are hugeTLB pages
102 (:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`),
103 the SLUB etc. memory allocators and various device drivers.
104 However in this interface, only huge/giga pages are made visible
107 A compound page tail (see description above).
109 this is an integral part of a HugeTLB page
111 hardware detected memory corruption on this page: don't touch the data!
113 no page frame exists at the requested address
115 identical memory pages dynamically shared between one or more processes
117 contiguous pages which construct transparent hugepages
119 balloon compaction page
121 zero page for pfn_zero or huge_zero page
123 page has not been accessed since it was marked idle (see
124 :ref:`Documentation/admin-guide/mm/idle_page_tracking.rst <idle_page_tracking>`).
125 Note that this flag may be stale in case the page was accessed via
126 a PTE. To make sure the flag is up-to-date one has to read
127 ``/sys/kernel/mm/page_idle/bitmap`` first.
129 IO related page flags
130 ---------------------
135 page has up-to-date data
136 ie. for file backed page: (in-memory data revision >= on-disk one)
138 page has been written to, hence contains new data
139 i.e. for file backed page: (in-memory data revision > on-disk one)
141 page is being synced to disk
143 LRU related page flags
144 ----------------------
147 page is in one of the LRU lists
149 page is in the active LRU list
151 page is in the unevictable (non-)LRU list It is somehow pinned and
152 not a candidate for LRU page reclaims, e.g. ramfs pages,
153 shmctl(SHM_LOCK) and mlock() memory segments
155 page has been referenced since last LRU list enqueue/requeue
157 page will be reclaimed soon after its pageout IO completed
161 a memory mapped page that is not part of a file
163 page is mapped to swap space, i.e. has an associated swap entry
165 page is backed by swap/RAM
167 The page-types tool in the tools/vm directory can be used to query the
170 Using pagemap to do something useful
171 ====================================
173 The general procedure for using pagemap to find out about a process' memory
174 usage goes like this:
176 1. Read ``/proc/pid/maps`` to determine which parts of the memory space are
178 2. Select the maps you are interested in -- all of them, or a particular
179 library, or the stack or the heap, etc.
180 3. Open ``/proc/pid/pagemap`` and seek to the pages you would like to examine.
181 4. Read a u64 for each page from pagemap.
182 5. Open ``/proc/kpagecount`` and/or ``/proc/kpageflags``. For each PFN you
183 just read, seek to that entry in the file, and read the data you want.
185 For example, to find the "unique set size" (USS), which is the amount of
186 memory that a process is using that is not shared with any other process,
187 you can go through every map in the process, find the PFNs, look those up
188 in kpagecount, and tally up the number of pages that are only referenced
194 Reading from any of the files will return -EINVAL if you are not starting
195 the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
196 into the file), or if the size of the read is not a multiple of 8 bytes.
198 Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
199 always 12 at most architectures). Since Linux 3.11 their meaning changes
200 after first clear of soft-dirty bits. Since Linux 4.2 they are used for
201 flags unconditionally.