--- /dev/null
+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
+ * Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
+ * Bits 56-60 zero
+ * 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
+ 23. BALLOON
+ 24. ZERO_PAGE
+
+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
+
+23. BALLOON
+ balloon compaction page
+
+24. ZERO_PAGE
+ zero page for pfn_zero or huge_zero page
+
+ [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 the tools/vm 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 sought an odd number of bytes
+into the file), or if the size of the read is not a multiple of 8 bytes.
Code Review / kvmfornfv.git / blobdiff