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diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt
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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    reserved for future use
+    * 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.