|author||Thomas Tuttle <firstname.lastname@example.org>||2008-06-05 22:46:59 -0700|
|committer||Linus Torvalds <email@example.com>||2008-06-06 11:29:14 -0700|
pagemap: add documentation for pagemap
Just a quick explanation of the pagemap interface from a userspace point of view, and an example of how to use it (in English, not code). Signed-off-by: Thomas Tuttle <firstname.lastname@example.org> Cc: Matt Mackall <email@example.com> Signed-off-by: Andrew Morton <firstname.lastname@example.org> Signed-off-by: Linus Torvalds <email@example.com>
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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 0-55 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
+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.