1pagemap, from the userspace perspective 2--------------------------------------- 3 4pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow 5userspace programs to examine the page tables and related information by 6reading files in /proc. 7 8There are three components to pagemap: 9 10 * /proc/pid/pagemap. This file lets a userspace process find out which 11 physical frame each virtual page is mapped to. It contains one 64-bit 12 value for each virtual page, containing the following data (from 13 fs/proc/task_mmu.c, above pagemap_read): 14 15 * Bits 0-54 page frame number (PFN) if present 16 * Bits 0-4 swap type if swapped 17 * Bits 5-54 swap offset if swapped 18 * Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt) 19 * Bits 56-60 zero 20 * Bit 61 page is file-page or shared-anon 21 * Bit 62 page swapped 22 * Bit 63 page present 23 24 If the page is not present but in swap, then the PFN contains an 25 encoding of the swap file number and the page's offset into the 26 swap. Unmapped pages return a null PFN. This allows determining 27 precisely which pages are mapped (or in swap) and comparing mapped 28 pages between processes. 29 30 Efficient users of this interface will use /proc/pid/maps to 31 determine which areas of memory are actually mapped and llseek to 32 skip over unmapped regions. 33 34 * /proc/kpagecount. This file contains a 64-bit count of the number of 35 times each page is mapped, indexed by PFN. 36 37 * /proc/kpageflags. This file contains a 64-bit set of flags for each 38 page, indexed by PFN. 39 40 The flags are (from fs/proc/page.c, above kpageflags_read): 41 42 0. LOCKED 43 1. ERROR 44 2. REFERENCED 45 3. UPTODATE 46 4. DIRTY 47 5. LRU 48 6. ACTIVE 49 7. SLAB 50 8. WRITEBACK 51 9. RECLAIM 52 10. BUDDY 53 11. MMAP 54 12. ANON 55 13. SWAPCACHE 56 14. SWAPBACKED 57 15. COMPOUND_HEAD 58 16. COMPOUND_TAIL 59 16. HUGE 60 18. UNEVICTABLE 61 19. HWPOISON 62 20. NOPAGE 63 21. KSM 64 22. THP 65 23. BALLOON 66 24. ZERO_PAGE 67 68Short descriptions to the page flags: 69 70 0. LOCKED 71 page is being locked for exclusive access, eg. by undergoing read/write IO 72 73 7. SLAB 74 page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator 75 When compound page is used, SLUB/SLQB will only set this flag on the head 76 page; SLOB will not flag it at all. 77 7810. BUDDY 79 a free memory block managed by the buddy system allocator 80 The buddy system organizes free memory in blocks of various orders. 81 An order N block has 2^N physically contiguous pages, with the BUDDY flag 82 set for and _only_ for the first page. 83 8415. COMPOUND_HEAD 8516. COMPOUND_TAIL 86 A compound page with order N consists of 2^N physically contiguous pages. 87 A compound page with order 2 takes the form of "HTTT", where H donates its 88 head page and T donates its tail page(s). The major consumers of compound 89 pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc. 90 memory allocators and various device drivers. However in this interface, 91 only huge/giga pages are made visible to end users. 9217. HUGE 93 this is an integral part of a HugeTLB page 94 9519. HWPOISON 96 hardware detected memory corruption on this page: don't touch the data! 97 9820. NOPAGE 99 no page frame exists at the requested address 100 10121. KSM 102 identical memory pages dynamically shared between one or more processes 103 10422. THP 105 contiguous pages which construct transparent hugepages 106 10723. BALLOON 108 balloon compaction page 109 11024. ZERO_PAGE 111 zero page for pfn_zero or huge_zero page 112 113 [IO related page flags] 114 1. ERROR IO error occurred 115 3. UPTODATE page has up-to-date data 116 ie. for file backed page: (in-memory data revision >= on-disk one) 117 4. DIRTY page has been written to, hence contains new data 118 ie. for file backed page: (in-memory data revision > on-disk one) 119 8. WRITEBACK page is being synced to disk 120 121 [LRU related page flags] 122 5. LRU page is in one of the LRU lists 123 6. ACTIVE page is in the active LRU list 12418. UNEVICTABLE page is in the unevictable (non-)LRU list 125 It is somehow pinned and not a candidate for LRU page reclaims, 126 eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments 127 2. REFERENCED page has been referenced since last LRU list enqueue/requeue 128 9. RECLAIM page will be reclaimed soon after its pageout IO completed 12911. MMAP a memory mapped page 13012. ANON a memory mapped page that is not part of a file 13113. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry 13214. SWAPBACKED page is backed by swap/RAM 133 134The page-types tool in the tools/vm directory can be used to query the 135above flags. 136 137Using pagemap to do something useful: 138 139The general procedure for using pagemap to find out about a process' memory 140usage goes like this: 141 142 1. Read /proc/pid/maps to determine which parts of the memory space are 143 mapped to what. 144 2. Select the maps you are interested in -- all of them, or a particular 145 library, or the stack or the heap, etc. 146 3. Open /proc/pid/pagemap and seek to the pages you would like to examine. 147 4. Read a u64 for each page from pagemap. 148 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just 149 read, seek to that entry in the file, and read the data you want. 150 151For example, to find the "unique set size" (USS), which is the amount of 152memory that a process is using that is not shared with any other process, 153you can go through every map in the process, find the PFNs, look those up 154in kpagecount, and tally up the number of pages that are only referenced 155once. 156 157Other notes: 158 159Reading from any of the files will return -EINVAL if you are not starting 160the read on an 8-byte boundary (e.g., if you sought an odd number of bytes 161into the file), or if the size of the read is not a multiple of 8 bytes. 162