mirror of
https://github.com/torvalds/linux.git
synced 2024-11-07 04:32:03 +00:00
ef421be741
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 <ttuttle@google.com> Cc: Matt Mackall <mpm@selenic.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
78 lines
2.8 KiB
Plaintext
78 lines
2.8 KiB
Plaintext
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
|
|
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.
|