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 * Bits 57-60 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 47 * ``/proc/kpageflags``. This file contains a 64-bit set of flags for each 48 page, indexed by PFN. 49 50 The flags are (from ``fs/proc/page.c``, above kpageflags_read): 51 52 0. LOCKED 53 1. ERROR 54 2. REFERENCED 55 3. UPTODATE 56 4. DIRTY 57 5. LRU 58 6. ACTIVE 59 7. SLAB 60 8. WRITEBACK 61 9. RECLAIM 62 10. BUDDY 63 11. MMAP 64 12. ANON 65 13. SWAPCACHE 66 14. SWAPBACKED 67 15. COMPOUND_HEAD 68 16. COMPOUND_TAIL 69 17. HUGE 70 18. UNEVICTABLE 71 19. HWPOISON 72 20. NOPAGE 73 21. KSM 74 22. THP 75 23. BALLOON 76 24. ZERO_PAGE 77 25. IDLE 78 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 81 CONFIG_MEMCG is set. 82 83Short descriptions to the page flags 84==================================== 85 860 - LOCKED 87 page is being locked for exclusive access, e.g. by undergoing read/write IO 887 - SLAB 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. 9210 - BUDDY 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. 9715 - COMPOUND_HEAD 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 105 to end users. 10616 - COMPOUND_TAIL 107 A compound page tail (see description above). 10817 - HUGE 109 this is an integral part of a HugeTLB page 11019 - HWPOISON 111 hardware detected memory corruption on this page: don't touch the data! 11220 - NOPAGE 113 no page frame exists at the requested address 11421 - KSM 115 identical memory pages dynamically shared between one or more processes 11622 - THP 117 contiguous pages which construct transparent hugepages 11823 - BALLOON 119 balloon compaction page 12024 - ZERO_PAGE 121 zero page for pfn_zero or huge_zero page 12225 - IDLE 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. 128 129IO related page flags 130--------------------- 131 1321 - ERROR 133 IO error occurred 1343 - UPTODATE 135 page has up-to-date data 136 ie. for file backed page: (in-memory data revision >= on-disk one) 1374 - DIRTY 138 page has been written to, hence contains new data 139 i.e. for file backed page: (in-memory data revision > on-disk one) 1408 - WRITEBACK 141 page is being synced to disk 142 143LRU related page flags 144---------------------- 145 1465 - LRU 147 page is in one of the LRU lists 1486 - ACTIVE 149 page is in the active LRU list 15018 - UNEVICTABLE 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 1542 - REFERENCED 155 page has been referenced since last LRU list enqueue/requeue 1569 - RECLAIM 157 page will be reclaimed soon after its pageout IO completed 15811 - MMAP 159 a memory mapped page 16012 - ANON 161 a memory mapped page that is not part of a file 16213 - SWAPCACHE 163 page is mapped to swap space, i.e. has an associated swap entry 16414 - SWAPBACKED 165 page is backed by swap/RAM 166 167The page-types tool in the tools/vm directory can be used to query the 168above flags. 169 170Using pagemap to do something useful 171==================================== 172 173The general procedure for using pagemap to find out about a process' memory 174usage goes like this: 175 176 1. Read ``/proc/pid/maps`` to determine which parts of the memory space are 177 mapped to what. 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. 184 185For example, to find the "unique set size" (USS), which is the amount of 186memory that a process is using that is not shared with any other process, 187you can go through every map in the process, find the PFNs, look those up 188in kpagecount, and tally up the number of pages that are only referenced 189once. 190 191Other notes 192=========== 193 194Reading from any of the files will return -EINVAL if you are not starting 195the read on an 8-byte boundary (e.g., if you sought an odd number of bytes 196into the file), or if the size of the read is not a multiple of 8 bytes. 197 198Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is 199always 12 at most architectures). Since Linux 3.11 their meaning changes 200after first clear of soft-dirty bits. Since Linux 4.2 they are used for 201flags unconditionally. 202