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 * Bits 55-60 page shift (page size = 1<<page shift) 19 * Bit 61 reserved for future use 20 * Bit 62 page swapped 21 * Bit 63 page present 22 23 If the page is not present but in swap, then the PFN contains an 24 encoding of the swap file number and the page's offset into the 25 swap. Unmapped pages return a null PFN. This allows determining 26 precisely which pages are mapped (or in swap) and comparing mapped 27 pages between processes. 28 29 Efficient users of this interface will use /proc/pid/maps to 30 determine which areas of memory are actually mapped and llseek to 31 skip over unmapped regions. 32 33 * /proc/kpagecount. This file contains a 64-bit count of the number of 34 times each page is mapped, indexed by PFN. 35 36 * /proc/kpageflags. This file contains a 64-bit set of flags for each 37 page, indexed by PFN. 38 39 The flags are (from fs/proc/page.c, above kpageflags_read): 40 41 0. LOCKED 42 1. ERROR 43 2. REFERENCED 44 3. UPTODATE 45 4. DIRTY 46 5. LRU 47 6. ACTIVE 48 7. SLAB 49 8. WRITEBACK 50 9. RECLAIM 51 10. BUDDY 52 11. MMAP 53 12. ANON 54 13. SWAPCACHE 55 14. SWAPBACKED 56 15. COMPOUND_HEAD 57 16. COMPOUND_TAIL 58 16. HUGE 59 18. UNEVICTABLE 60 19. HWPOISON 61 20. NOPAGE 62 21. KSM 63 22. THP 64 65Short descriptions to the page flags: 66 67 0. LOCKED 68 page is being locked for exclusive access, eg. by undergoing read/write IO 69 70 7. SLAB 71 page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator 72 When compound page is used, SLUB/SLQB will only set this flag on the head 73 page; SLOB will not flag it at all. 74 7510. BUDDY 76 a free memory block managed by the buddy system allocator 77 The buddy system organizes free memory in blocks of various orders. 78 An order N block has 2^N physically contiguous pages, with the BUDDY flag 79 set for and _only_ for the first page. 80 8115. COMPOUND_HEAD 8216. COMPOUND_TAIL 83 A compound page with order N consists of 2^N physically contiguous pages. 84 A compound page with order 2 takes the form of "HTTT", where H donates its 85 head page and T donates its tail page(s). The major consumers of compound 86 pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc. 87 memory allocators and various device drivers. However in this interface, 88 only huge/giga pages are made visible to end users. 8917. HUGE 90 this is an integral part of a HugeTLB page 91 9219. HWPOISON 93 hardware detected memory corruption on this page: don't touch the data! 94 9520. NOPAGE 96 no page frame exists at the requested address 97 9821. KSM 99 identical memory pages dynamically shared between one or more processes 100 10122. THP 102 contiguous pages which construct transparent hugepages 103 104 [IO related page flags] 105 1. ERROR IO error occurred 106 3. UPTODATE page has up-to-date data 107 ie. for file backed page: (in-memory data revision >= on-disk one) 108 4. DIRTY page has been written to, hence contains new data 109 ie. for file backed page: (in-memory data revision > on-disk one) 110 8. WRITEBACK page is being synced to disk 111 112 [LRU related page flags] 113 5. LRU page is in one of the LRU lists 114 6. ACTIVE page is in the active LRU list 11518. UNEVICTABLE page is in the unevictable (non-)LRU list 116 It is somehow pinned and not a candidate for LRU page reclaims, 117 eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments 118 2. REFERENCED page has been referenced since last LRU list enqueue/requeue 119 9. RECLAIM page will be reclaimed soon after its pageout IO completed 12011. MMAP a memory mapped page 12112. ANON a memory mapped page that is not part of a file 12213. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry 12314. SWAPBACKED page is backed by swap/RAM 124 125The page-types tool in this directory can be used to query the above flags. 126 127Using pagemap to do something useful: 128 129The general procedure for using pagemap to find out about a process' memory 130usage goes like this: 131 132 1. Read /proc/pid/maps to determine which parts of the memory space are 133 mapped to what. 134 2. Select the maps you are interested in -- all of them, or a particular 135 library, or the stack or the heap, etc. 136 3. Open /proc/pid/pagemap and seek to the pages you would like to examine. 137 4. Read a u64 for each page from pagemap. 138 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just 139 read, seek to that entry in the file, and read the data you want. 140 141For example, to find the "unique set size" (USS), which is the amount of 142memory that a process is using that is not shared with any other process, 143you can go through every map in the process, find the PFNs, look those up 144in kpagecount, and tally up the number of pages that are only referenced 145once. 146 147Other notes: 148 149Reading from any of the files will return -EINVAL if you are not starting 150the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes 151into the file), or if the size of the read is not a multiple of 8 bytes. 152