152 lines
5.6 KiB
Text
152 lines
5.6 KiB
Text
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
|
|
|
|
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 sought an odd number of bytes
|
|
into the file), or if the size of the read is not a multiple of 8 bytes.
|