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 47The page-types tool in the tools/vm 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 page is being locked for exclusive access, e.g. by undergoing read/write IO 927 - SLAB 93 page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator 94 When compound page is used, SLUB/SLQB will only set this flag on the head 95 page; SLOB will not flag it at all. 9610 - BUDDY 97 a free memory block managed by the buddy system allocator 98 The buddy system organizes free memory in blocks of various orders. 99 An order N block has 2^N physically contiguous pages, with the BUDDY flag 100 set for and _only_ for the first page. 10115 - COMPOUND_HEAD 102 A compound page with order N consists of 2^N physically contiguous pages. 103 A compound page with order 2 takes the form of "HTTT", where H donates its 104 head page and T donates its tail page(s). The major consumers of compound 105 pages are hugeTLB pages 106 (:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`), 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 page is logically offline 12424 - ZERO_PAGE 125 zero page for pfn_zero or huge_zero page 12625 - IDLE 127 page has not been accessed since it was marked idle (see 128 :ref:`Documentation/admin-guide/mm/idle_page_tracking.rst <idle_page_tracking>`). 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 page is in use as a page table 134 135IO related page flags 136--------------------- 137 1381 - ERROR 139 IO error occurred 1403 - UPTODATE 141 page has up-to-date data 142 ie. for file backed page: (in-memory data revision >= on-disk one) 1434 - DIRTY 144 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 page is being synced to disk 148 149LRU related page flags 150---------------------- 151 1525 - LRU 153 page is in one of the LRU lists 1546 - ACTIVE 155 page is in the active LRU list 15618 - UNEVICTABLE 157 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 page has been referenced since last LRU list enqueue/requeue 1629 - RECLAIM 163 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 page is mapped to swap space, i.e. has an associated swap entry 17014 - SWAPBACKED 171 page is backed by swap/RAM 172 173The page-types tool in the tools/vm 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 197Other notes 198=========== 199 200Reading from any of the files will return -EINVAL if you are not starting 201the read on an 8-byte boundary (e.g., if you sought an odd number of bytes 202into the file), or if the size of the read is not a multiple of 8 bytes. 203 204Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is 205always 12 at most architectures). Since Linux 3.11 their meaning changes 206after first clear of soft-dirty bits. Since Linux 4.2 they are used for 207flags unconditionally. 208