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This is an implementation of Andrew's proposal to extend the pagemap file bits to report what is missing about tasks' working set. The problem with the working set detection is multilateral. In the criu (checkpoint/restore) project we dump the tasks' memory into image files and to do it properly we need to detect which pages inside mappings are really in use. The mincore syscall I though could help with this did not. First, it doesn't report swapped pages, thus we cannot find out which parts of anonymous mappings to dump. Next, it does report pages from page cache as present even if they are not mapped, and it doesn't make that has not been cow-ed. Note, that issue with swap pages is critical -- we must dump swap pages to image file. But the issues with file pages are optimization -- we can take all file pages to image, this would be correct, but if we know that a page is not mapped or not cow-ed, we can remove them from dump file. The dump would still be self-consistent, though significantly smaller in size (up to 10 times smaller on real apps). Andrew noticed, that the proc pagemap file solved 2 of 3 above issues -- it reports whether a page is present or swapped and it doesn't report not mapped page cache pages. But, it doesn't distinguish cow-ed file pages from not cow-ed. I would like to make the last unused bit in this file to report whether the page mapped into respective pte is PageAnon or not. [comment stolen from Pavel Emelyanov's v1 patch] Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org> Cc: Pavel Emelyanov <xemul@parallels.com> Cc: Matt Mackall <mpm@selenic.com> Cc: Hugh Dickins <hughd@google.com> Cc: Rik van Riel <riel@redhat.com> Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
152 lines
5.6 KiB
Plaintext
152 lines
5.6 KiB
Plaintext
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
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userspace programs to examine the page tables and related information by
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reading files in /proc.
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There are three components to pagemap:
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* /proc/pid/pagemap. This file lets a userspace process find out which
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physical frame each virtual page is mapped to. It contains one 64-bit
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value for each virtual page, containing the following data (from
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fs/proc/task_mmu.c, above pagemap_read):
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* Bits 0-54 page frame number (PFN) if present
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* Bits 0-4 swap type if swapped
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* Bits 5-54 swap offset if swapped
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* Bits 55-60 page shift (page size = 1<<page shift)
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* Bit 61 page is file-page or shared-anon
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* Bit 62 page swapped
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* Bit 63 page present
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If the page is not present but in swap, then the PFN contains an
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encoding of the swap file number and the page's offset into the
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swap. Unmapped pages return a null PFN. This allows determining
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precisely which pages are mapped (or in swap) and comparing mapped
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pages between processes.
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Efficient users of this interface will use /proc/pid/maps to
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determine which areas of memory are actually mapped and llseek to
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skip over unmapped regions.
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* /proc/kpagecount. This file contains a 64-bit count of the number of
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times each page is mapped, indexed by PFN.
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* /proc/kpageflags. This file contains a 64-bit set of flags for each
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page, indexed by PFN.
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The flags are (from fs/proc/page.c, above kpageflags_read):
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0. LOCKED
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1. ERROR
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2. REFERENCED
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3. UPTODATE
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4. DIRTY
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5. LRU
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6. ACTIVE
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7. SLAB
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8. WRITEBACK
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9. RECLAIM
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10. BUDDY
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11. MMAP
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12. ANON
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13. SWAPCACHE
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14. SWAPBACKED
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15. COMPOUND_HEAD
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16. COMPOUND_TAIL
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16. HUGE
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18. UNEVICTABLE
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19. HWPOISON
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20. NOPAGE
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21. KSM
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22. THP
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Short descriptions to the page flags:
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0. LOCKED
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page is being locked for exclusive access, eg. by undergoing read/write IO
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7. SLAB
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page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
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When compound page is used, SLUB/SLQB will only set this flag on the head
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page; SLOB will not flag it at all.
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10. BUDDY
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a free memory block managed by the buddy system allocator
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The buddy system organizes free memory in blocks of various orders.
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An order N block has 2^N physically contiguous pages, with the BUDDY flag
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set for and _only_ for the first page.
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15. COMPOUND_HEAD
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16. COMPOUND_TAIL
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A compound page with order N consists of 2^N physically contiguous pages.
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A compound page with order 2 takes the form of "HTTT", where H donates its
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head page and T donates its tail page(s). The major consumers of compound
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pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
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memory allocators and various device drivers. However in this interface,
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only huge/giga pages are made visible to end users.
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17. HUGE
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this is an integral part of a HugeTLB page
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19. HWPOISON
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hardware detected memory corruption on this page: don't touch the data!
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20. NOPAGE
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no page frame exists at the requested address
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21. KSM
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identical memory pages dynamically shared between one or more processes
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22. THP
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contiguous pages which construct transparent hugepages
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[IO related page flags]
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1. ERROR IO error occurred
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3. UPTODATE page has up-to-date data
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ie. for file backed page: (in-memory data revision >= on-disk one)
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4. DIRTY page has been written to, hence contains new data
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ie. for file backed page: (in-memory data revision > on-disk one)
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8. WRITEBACK page is being synced to disk
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[LRU related page flags]
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5. LRU page is in one of the LRU lists
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6. ACTIVE page is in the active LRU list
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18. UNEVICTABLE page is in the unevictable (non-)LRU list
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It is somehow pinned and not a candidate for LRU page reclaims,
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eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
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2. REFERENCED page has been referenced since last LRU list enqueue/requeue
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9. RECLAIM page will be reclaimed soon after its pageout IO completed
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11. MMAP a memory mapped page
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12. ANON a memory mapped page that is not part of a file
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13. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry
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14. SWAPBACKED page is backed by swap/RAM
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The page-types tool in this directory can be used to query the above flags.
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Using pagemap to do something useful:
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The general procedure for using pagemap to find out about a process' memory
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usage goes like this:
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1. Read /proc/pid/maps to determine which parts of the memory space are
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mapped to what.
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2. Select the maps you are interested in -- all of them, or a particular
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library, or the stack or the heap, etc.
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3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
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4. Read a u64 for each page from pagemap.
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5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just
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read, seek to that entry in the file, and read the data you want.
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For example, to find the "unique set size" (USS), which is the amount of
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memory that a process is using that is not shared with any other process,
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you can go through every map in the process, find the PFNs, look those up
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in kpagecount, and tally up the number of pages that are only referenced
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once.
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Other notes:
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Reading from any of the files will return -EINVAL if you are not starting
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the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
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into the file), or if the size of the read is not a multiple of 8 bytes.
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