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-pagemap, from the userspace perspective
----------------------------------------
-
-pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
-userspace programs to examine the page tables and related information by
-reading files in /proc.
-
-There are three components to pagemap:
-
- * /proc/pid/pagemap. This file lets a userspace process find out which
- physical frame each virtual page is mapped to. It contains one 64-bit
- value for each virtual page, containing the following data (from
- fs/proc/task_mmu.c, above pagemap_read):
-
- * Bits 0-54 page frame number (PFN) if present
- * Bits 0-4 swap type if swapped
- * Bits 5-54 swap offset if swapped
- * Bits 55-60 page shift (page size = 1<<page shift)
- * Bit 61 page is file-page or shared-anon
- * Bit 62 page swapped
- * Bit 63 page present
-
- If the page is not present but in swap, then the PFN contains an
- encoding of the swap file number and the page's offset into the
- swap. Unmapped pages return a null PFN. This allows determining
- precisely which pages are mapped (or in swap) and comparing mapped
- pages between processes.
-
- Efficient users of this interface will use /proc/pid/maps to
- determine which areas of memory are actually mapped and llseek to
- skip over unmapped regions.
-
- * /proc/kpagecount. This file contains a 64-bit count of the number of
- times each page is mapped, indexed by PFN.
-
- * /proc/kpageflags. This file contains a 64-bit set of flags for each
- page, indexed by PFN.
-
- The flags are (from fs/proc/page.c, above kpageflags_read):
-
- 0. LOCKED
- 1. ERROR
- 2. REFERENCED
- 3. UPTODATE
- 4. DIRTY
- 5. LRU
- 6. ACTIVE
- 7. SLAB
- 8. WRITEBACK
- 9. RECLAIM
- 10. BUDDY
- 11. MMAP
- 12. ANON
- 13. SWAPCACHE
- 14. SWAPBACKED
- 15. COMPOUND_HEAD
- 16. COMPOUND_TAIL
- 16. HUGE
- 18. UNEVICTABLE
- 19. HWPOISON
- 20. NOPAGE
- 21. KSM
- 22. THP
-
-Short descriptions to the page flags:
-
- 0. LOCKED
- page is being locked for exclusive access, eg. by undergoing read/write IO
-
- 7. SLAB
- page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
- When compound page is used, SLUB/SLQB will only set this flag on the head
- page; SLOB will not flag it at all.
-
-10. BUDDY
- a free memory block managed by the buddy system allocator
- The buddy system organizes free memory in blocks of various orders.
- An order N block has 2^N physically contiguous pages, with the BUDDY flag
- set for and _only_ for the first page.
-
-15. COMPOUND_HEAD
-16. COMPOUND_TAIL
- A compound page with order N consists of 2^N physically contiguous pages.
- A compound page with order 2 takes the form of "HTTT", where H donates its
- head page and T donates its tail page(s). The major consumers of compound
- pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
- memory allocators and various device drivers. However in this interface,
- only huge/giga pages are made visible to end users.
-17. HUGE
- this is an integral part of a HugeTLB page
-
-19. HWPOISON
- hardware detected memory corruption on this page: don't touch the data!
-
-20. NOPAGE
- no page frame exists at the requested address
-
-21. KSM
- identical memory pages dynamically shared between one or more processes
-
-22. THP
- contiguous pages which construct transparent hugepages
-
- [IO related page flags]
- 1. ERROR IO error occurred
- 3. UPTODATE page has up-to-date data
- ie. for file backed page: (in-memory data revision >= on-disk one)
- 4. DIRTY page has been written to, hence contains new data
- ie. for file backed page: (in-memory data revision > on-disk one)
- 8. WRITEBACK page is being synced to disk
-
- [LRU related page flags]
- 5. LRU page is in one of the LRU lists
- 6. ACTIVE page is in the active LRU list
-18. UNEVICTABLE page is in the unevictable (non-)LRU list
- It is somehow pinned and not a candidate for LRU page reclaims,
- eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
- 2. REFERENCED page has been referenced since last LRU list enqueue/requeue
- 9. RECLAIM page will be reclaimed soon after its pageout IO completed
-11. MMAP a memory mapped page
-12. ANON a memory mapped page that is not part of a file
-13. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry
-14. SWAPBACKED page is backed by swap/RAM
-
-The page-types tool in this directory can be used to query the above flags.
-
-Using pagemap to do something useful:
-
-The general procedure for using pagemap to find out about a process' memory
-usage goes like this:
-
- 1. Read /proc/pid/maps to determine which parts of the memory space are
- mapped to what.
- 2. Select the maps you are interested in -- all of them, or a particular
- library, or the stack or the heap, etc.
- 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
- 4. Read a u64 for each page from pagemap.
- 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just
- read, seek to that entry in the file, and read the data you want.
-
-For example, to find the "unique set size" (USS), which is the amount of
-memory that a process is using that is not shared with any other process,
-you can go through every map in the process, find the PFNs, look those up
-in kpagecount, and tally up the number of pages that are only referenced
-once.
-
-Other notes:
-
-Reading from any of the files will return -EINVAL if you are not starting
-the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
-into the file), or if the size of the read is not a multiple of 8 bytes.