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 0-55 page frame number (PFN) if present 16 * Bits 0-4 swap type if swapped 17 * Bits 5-55 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/proc_misc, 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 53Using pagemap to do something useful: 54 55The general procedure for using pagemap to find out about a process' memory 56usage goes like this: 57 58 1. Read /proc/pid/maps to determine which parts of the memory space are 59 mapped to what. 60 2. Select the maps you are interested in -- all of them, or a particular 61 library, or the stack or the heap, etc. 62 3. Open /proc/pid/pagemap and seek to the pages you would like to examine. 63 4. Read a u64 for each page from pagemap. 64 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just 65 read, seek to that entry in the file, and read the data you want. 66 67For example, to find the "unique set size" (USS), which is the amount of 68memory that a process is using that is not shared with any other process, 69you can go through every map in the process, find the PFNs, look those up 70in kpagecount, and tally up the number of pages that are only referenced 71once. 72 73Other notes: 74 75Reading from any of the files will return -EINVAL if you are not starting 76the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes 77into the file), or if the size of the read is not a multiple of 8 bytes. 78