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 58-60 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 The page is being locked for exclusive access, e.g. by undergoing read/write 94 IO. 957 - SLAB 96 The page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator. 97 When compound page is used, SLUB/SLQB will only set this flag on the head 98 page; SLOB will not flag it at all. 9910 - BUDDY 100 A free memory block managed by the buddy system allocator. 101 The buddy system organizes free memory in blocks of various orders. 102 An order N block has 2^N physically contiguous pages, with the BUDDY flag 103 set for and _only_ for the first page. 10415 - COMPOUND_HEAD 105 A compound page with order N consists of 2^N physically contiguous pages. 106 A compound page with order 2 takes the form of "HTTT", where H donates its 107 head page and T donates its tail page(s). The major consumers of compound 108 pages are hugeTLB pages 109 (:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`), 110 the SLUB etc. memory allocators and various device drivers. 111 However in this interface, only huge/giga pages are made visible 112 to end users. 11316 - COMPOUND_TAIL 114 A compound page tail (see description above). 11517 - HUGE 116 This is an integral part of a HugeTLB page. 11719 - HWPOISON 118 Hardware detected memory corruption on this page: don't touch the data! 11920 - NOPAGE 120 No page frame exists at the requested address. 12121 - KSM 122 Identical memory pages dynamically shared between one or more processes. 12322 - THP 124 Contiguous pages which construct transparent hugepages. 12523 - OFFLINE 126 The page is logically offline. 12724 - ZERO_PAGE 128 Zero page for pfn_zero or huge_zero page. 12925 - IDLE 130 The page has not been accessed since it was marked idle (see 131 :ref:`Documentation/admin-guide/mm/idle_page_tracking.rst <idle_page_tracking>`). 132 Note that this flag may be stale in case the page was accessed via 133 a PTE. To make sure the flag is up-to-date one has to read 134 ``/sys/kernel/mm/page_idle/bitmap`` first. 13526 - PGTABLE 136 The page is in use as a page table. 137 138IO related page flags 139--------------------- 140 1411 - ERROR 142 IO error occurred. 1433 - UPTODATE 144 The page has up-to-date data. 145 ie. for file backed page: (in-memory data revision >= on-disk one) 1464 - DIRTY 147 The page has been written to, hence contains new data. 148 i.e. for file backed page: (in-memory data revision > on-disk one) 1498 - WRITEBACK 150 The page is being synced to disk. 151 152LRU related page flags 153---------------------- 154 1555 - LRU 156 The page is in one of the LRU lists. 1576 - ACTIVE 158 The page is in the active LRU list. 15918 - UNEVICTABLE 160 The page is in the unevictable (non-)LRU list It is somehow pinned and 161 not a candidate for LRU page reclaims, e.g. ramfs pages, 162 shmctl(SHM_LOCK) and mlock() memory segments. 1632 - REFERENCED 164 The page has been referenced since last LRU list enqueue/requeue. 1659 - RECLAIM 166 The page will be reclaimed soon after its pageout IO completed. 16711 - MMAP 168 A memory mapped page. 16912 - ANON 170 A memory mapped page that is not part of a file. 17113 - SWAPCACHE 172 The page is mapped to swap space, i.e. has an associated swap entry. 17314 - SWAPBACKED 174 The page is backed by swap/RAM. 175 176The page-types tool in the tools/vm directory can be used to query the 177above flags. 178 179Using pagemap to do something useful 180==================================== 181 182The general procedure for using pagemap to find out about a process' memory 183usage goes like this: 184 185 1. Read ``/proc/pid/maps`` to determine which parts of the memory space are 186 mapped to what. 187 2. Select the maps you are interested in -- all of them, or a particular 188 library, or the stack or the heap, etc. 189 3. Open ``/proc/pid/pagemap`` and seek to the pages you would like to examine. 190 4. Read a u64 for each page from pagemap. 191 5. Open ``/proc/kpagecount`` and/or ``/proc/kpageflags``. For each PFN you 192 just read, seek to that entry in the file, and read the data you want. 193 194For example, to find the "unique set size" (USS), which is the amount of 195memory that a process is using that is not shared with any other process, 196you can go through every map in the process, find the PFNs, look those up 197in kpagecount, and tally up the number of pages that are only referenced 198once. 199 200Exceptions for Shared Memory 201============================ 202 203Page table entries for shared pages are cleared when the pages are zapped or 204swapped out. This makes swapped out pages indistinguishable from never-allocated 205ones. 206 207In kernel space, the swap location can still be retrieved from the page cache. 208However, values stored only on the normal PTE get lost irretrievably when the 209page is swapped out (i.e. SOFT_DIRTY). 210 211In user space, whether the page is present, swapped or none can be deduced with 212the help of lseek and/or mincore system calls. 213 214lseek() can differentiate between accessed pages (present or swapped out) and 215holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where 216the pages are backed. For anonymous shared pages, the file can be found in 217``/proc/pid/map_files/``. 218 219mincore() can differentiate between pages in memory (present, including swap 220cache) and out of memory (swapped out or none/non-allocated). 221 222Other notes 223=========== 224 225Reading from any of the files will return -EINVAL if you are not starting 226the read on an 8-byte boundary (e.g., if you sought an odd number of bytes 227into the file), or if the size of the read is not a multiple of 8 bytes. 228 229Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is 230always 12 at most architectures). Since Linux 3.11 their meaning changes 231after first clear of soft-dirty bits. Since Linux 4.2 they are used for 232flags unconditionally. 233