pagemap: add documentation for pagemap

pagemap, from the userspace perspective

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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 0-55  page frame number (PFN) if present

    * Bits 0-4   swap type if swapped

    * Bits 5-55  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/proc_misc, 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

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.