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 * Bit 58 pte is a guard region (since 6.15) (see madvise (2) man page) 25 * Bits 59-60 zero 26 * Bit 61 page is file-page or shared-anon (since 3.5) 27 * Bit 62 page swapped 28 * Bit 63 page present 29 30 Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs. 31 In 4.0 and 4.1 opens by unprivileged fail with -EPERM. Starting from 32 4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN. 33 Reason: information about PFNs helps in exploiting Rowhammer vulnerability. 34 35 If the page is not present but in swap, then the PFN contains an 36 encoding of the swap file number and the page's offset into the 37 swap. Unmapped pages return a null PFN. This allows determining 38 precisely which pages are mapped (or in swap) and comparing mapped 39 pages between processes. 40 41 Traditionally, bit 56 indicates that a page is mapped exactly once and bit 42 56 is clear when a page is mapped multiple times, even when mapped in the 43 same process multiple times. In some kernel configurations, the semantics 44 for pages part of a larger allocation (e.g., THP) can differ: bit 56 is set 45 if all pages part of the corresponding large allocation are *certainly* 46 mapped in the same process, even if the page is mapped multiple times in that 47 process. Bit 56 is clear when any page page of the larger allocation 48 is *maybe* mapped in a different process. In some cases, a large allocation 49 might be treated as "maybe mapped by multiple processes" even though this 50 is no longer the case. 51 52 Efficient users of this interface will use ``/proc/pid/maps`` to 53 determine which areas of memory are actually mapped and llseek to 54 skip over unmapped regions. 55 56 * ``/proc/kpagecount``. This file contains a 64-bit count of the number of 57 times each page is mapped, indexed by PFN. Some kernel configurations do 58 not track the precise number of times a page part of a larger allocation 59 (e.g., THP) is mapped. In these configurations, the average number of 60 mappings per page in this larger allocation is returned instead. However, 61 if any page of the large allocation is mapped, the returned value will 62 be at least 1. 63 64The page-types tool in the tools/mm directory can be used to query the 65number of times a page is mapped. 66 67 * ``/proc/kpageflags``. This file contains a 64-bit set of flags for each 68 page, indexed by PFN. 69 70 The flags are (from ``fs/proc/page.c``, above kpageflags_read): 71 72 0. LOCKED 73 1. ERROR 74 2. REFERENCED 75 3. UPTODATE 76 4. DIRTY 77 5. LRU 78 6. ACTIVE 79 7. SLAB 80 8. WRITEBACK 81 9. RECLAIM 82 10. BUDDY 83 11. MMAP 84 12. ANON 85 13. SWAPCACHE 86 14. SWAPBACKED 87 15. COMPOUND_HEAD 88 16. COMPOUND_TAIL 89 17. HUGE 90 18. UNEVICTABLE 91 19. HWPOISON 92 20. NOPAGE 93 21. KSM 94 22. THP 95 23. OFFLINE 96 24. ZERO_PAGE 97 25. IDLE 98 26. PGTABLE 99 100 * ``/proc/kpagecgroup``. This file contains a 64-bit inode number of the 101 memory cgroup each page is charged to, indexed by PFN. Only available when 102 CONFIG_MEMCG is set. 103 104Short descriptions to the page flags 105==================================== 106 1070 - LOCKED 108 The page is being locked for exclusive access, e.g. by undergoing read/write 109 IO. 1107 - SLAB 111 The page is managed by the SLAB/SLUB kernel memory allocator. 112 When compound page is used, either will only set this flag on the head 113 page. 11410 - BUDDY 115 A free memory block managed by the buddy system allocator. 116 The buddy system organizes free memory in blocks of various orders. 117 An order N block has 2^N physically contiguous pages, with the BUDDY flag 118 set for all pages. 119 Before 4.6 only the first page of the block had the flag set. 12015 - COMPOUND_HEAD 121 A compound page with order N consists of 2^N physically contiguous pages. 122 A compound page with order 2 takes the form of "HTTT", where H donates its 123 head page and T donates its tail page(s). The major consumers of compound 124 pages are hugeTLB pages (Documentation/admin-guide/mm/hugetlbpage.rst), 125 the SLUB etc. memory allocators and various device drivers. 126 However in this interface, only huge/giga pages are made visible 127 to end users. 12816 - COMPOUND_TAIL 129 A compound page tail (see description above). 13017 - HUGE 131 This is an integral part of a HugeTLB page. 13219 - HWPOISON 133 Hardware detected memory corruption on this page: don't touch the data! 13420 - NOPAGE 135 No page frame exists at the requested address. 13621 - KSM 137 Identical memory pages dynamically shared between one or more processes. 13822 - THP 139 Contiguous pages which construct THP of any size and mapped by any granularity. 14023 - OFFLINE 141 The page is logically offline. 14224 - ZERO_PAGE 143 Zero page for pfn_zero or huge_zero page. 14425 - IDLE 145 The page has not been accessed since it was marked idle (see 146 Documentation/admin-guide/mm/idle_page_tracking.rst). 147 Note that this flag may be stale in case the page was accessed via 148 a PTE. To make sure the flag is up-to-date one has to read 149 ``/sys/kernel/mm/page_idle/bitmap`` first. 15026 - PGTABLE 151 The page is in use as a page table. 152 153IO related page flags 154--------------------- 155 1561 - ERROR 157 IO error occurred. 1583 - UPTODATE 159 The page has up-to-date data. 160 ie. for file backed page: (in-memory data revision >= on-disk one) 1614 - DIRTY 162 The page has been written to, hence contains new data. 163 i.e. for file backed page: (in-memory data revision > on-disk one) 1648 - WRITEBACK 165 The page is being synced to disk. 166 167LRU related page flags 168---------------------- 169 1705 - LRU 171 The page is in one of the LRU lists. 1726 - ACTIVE 173 The page is in the active LRU list. 17418 - UNEVICTABLE 175 The page is in the unevictable (non-)LRU list It is somehow pinned and 176 not a candidate for LRU page reclaims, e.g. ramfs pages, 177 shmctl(SHM_LOCK) and mlock() memory segments. 1782 - REFERENCED 179 The page has been referenced since last LRU list enqueue/requeue. 1809 - RECLAIM 181 The page will be reclaimed soon after its pageout IO completed. 18211 - MMAP 183 A memory mapped page. 18412 - ANON 185 A memory mapped page that is not part of a file. 18613 - SWAPCACHE 187 The page is mapped to swap space, i.e. has an associated swap entry. 18814 - SWAPBACKED 189 The page is backed by swap/RAM. 190 191The page-types tool in the tools/mm directory can be used to query the 192above flags. 193 194Exceptions for Shared Memory 195============================ 196 197Page table entries for shared pages are cleared when the pages are zapped or 198swapped out. This makes swapped out pages indistinguishable from never-allocated 199ones. 200 201In kernel space, the swap location can still be retrieved from the page cache. 202However, values stored only on the normal PTE get lost irretrievably when the 203page is swapped out (i.e. SOFT_DIRTY). 204 205In user space, whether the page is present, swapped or none can be deduced with 206the help of lseek and/or mincore system calls. 207 208lseek() can differentiate between accessed pages (present or swapped out) and 209holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where 210the pages are backed. For anonymous shared pages, the file can be found in 211``/proc/pid/map_files/``. 212 213mincore() can differentiate between pages in memory (present, including swap 214cache) and out of memory (swapped out or none/non-allocated). 215 216Other notes 217=========== 218 219Reading from any of the files will return -EINVAL if you are not starting 220the read on an 8-byte boundary (e.g., if you sought an odd number of bytes 221into the file), or if the size of the read is not a multiple of 8 bytes. 222 223Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is 224always 12 at most architectures). Since Linux 3.11 their meaning changes 225after first clear of soft-dirty bits. Since Linux 4.2 they are used for 226flags unconditionally. 227 228Pagemap Scan IOCTL 229================== 230 231The ``PAGEMAP_SCAN`` IOCTL on the pagemap file can be used to get or optionally 232clear the info about page table entries. The following operations are supported 233in this IOCTL: 234 235- Scan the address range and get the memory ranges matching the provided criteria. 236 This is performed when the output buffer is specified. 237- Write-protect the pages. The ``PM_SCAN_WP_MATCHING`` is used to write-protect 238 the pages of interest. The ``PM_SCAN_CHECK_WPASYNC`` aborts the operation if 239 non-Async Write Protected pages are found. The ``PM_SCAN_WP_MATCHING`` can be 240 used with or without ``PM_SCAN_CHECK_WPASYNC``. 241- Both of those operations can be combined into one atomic operation where we can 242 get and write protect the pages as well. 243 244Following flags about pages are currently supported: 245 246- ``PAGE_IS_WPALLOWED`` - Page has async-write-protection enabled 247- ``PAGE_IS_WRITTEN`` - Page has been written to from the time it was write protected 248- ``PAGE_IS_FILE`` - Page is file backed 249- ``PAGE_IS_PRESENT`` - Page is present in the memory 250- ``PAGE_IS_SWAPPED`` - Page is in swapped 251- ``PAGE_IS_PFNZERO`` - Page has zero PFN 252- ``PAGE_IS_HUGE`` - Page is PMD-mapped THP or Hugetlb backed 253- ``PAGE_IS_SOFT_DIRTY`` - Page is soft-dirty 254- ``PAGE_IS_GUARD`` - Page is a part of a guard region 255 256The ``struct pm_scan_arg`` is used as the argument of the IOCTL. 257 258 1. The size of the ``struct pm_scan_arg`` must be specified in the ``size`` 259 field. This field will be helpful in recognizing the structure if extensions 260 are done later. 261 2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_WP_MATCHING`` 262 and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get 263 operation is optionally performed depending upon if the output buffer is 264 provided or not. 265 3. The range is specified through ``start`` and ``end``. 266 4. The walk can abort before visiting the complete range such as the user buffer 267 can get full etc. The walk ending address is specified in``end_walk``. 268 5. The output buffer of ``struct page_region`` array and size is specified in 269 ``vec`` and ``vec_len``. 270 6. The optional maximum requested pages are specified in the ``max_pages``. 271 7. The masks are specified in ``category_mask``, ``category_anyof_mask``, 272 ``category_inverted`` and ``return_mask``. 273 274Find pages which have been written and WP them as well:: 275 276 struct pm_scan_arg arg = { 277 .size = sizeof(arg), 278 .flags = PM_SCAN_CHECK_WPASYNC | PM_SCAN_CHECK_WPASYNC, 279 .. 280 .category_mask = PAGE_IS_WRITTEN, 281 .return_mask = PAGE_IS_WRITTEN, 282 }; 283 284Find pages which have been written, are file backed, not swapped and either 285present or huge:: 286 287 struct pm_scan_arg arg = { 288 .size = sizeof(arg), 289 .flags = 0, 290 .. 291 .category_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED, 292 .category_inverted = PAGE_IS_SWAPPED, 293 .category_anyof_mask = PAGE_IS_PRESENT | PAGE_IS_HUGE, 294 .return_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED | 295 PAGE_IS_PRESENT | PAGE_IS_HUGE, 296 }; 297 298The ``PAGE_IS_WRITTEN`` flag can be considered as a better-performing alternative 299of soft-dirty flag. It doesn't get affected by VMA merging of the kernel and hence 300the user can find the true soft-dirty pages in case of normal pages. (There may 301still be extra dirty pages reported for THP or Hugetlb pages.) 302 303"PAGE_IS_WRITTEN" category is used with uffd write protect-enabled ranges to 304implement memory dirty tracking in userspace: 305 306 1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall. 307 2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features 308 are set by ``UFFDIO_API`` IOCTL. 309 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode 310 through ``UFFDIO_REGISTER`` IOCTL. 311 4. Then any part of the registered memory or the whole memory region must 312 be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_WP_MATCHING`` 313 or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the 314 same operation. The former is better in terms of performance. 315 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which 316 have been written to since they were last marked and/or optionally write protect 317 the pages as well. 318