@@ -1,13 +1,16 @@
-pagemap, from the userspace perspective
----------------------------------------
+.. _pagemap:
+
+======================================
+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.
+reading files in ``/proc``.
There are four components to pagemap:
- * /proc/pid/pagemap. This file lets a userspace process find out which
+ * ``/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):
@@ -37,24 +40,24 @@ There are four components to pagemap:
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
+ * ``/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
+ * ``/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
+ 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
@@ -72,98 +75,108 @@ There are four components to pagemap:
24. ZERO_PAGE
25. IDLE
- * /proc/kpagecgroup. This file contains a 64-bit inode number of the
+ * ``/proc/kpagecgroup``. This file contains a 64-bit inode number of the
memory cgroup each page is charged to, indexed by PFN. Only available when
CONFIG_MEMCG is set.
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
+=====================================
+
+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
+15 - COMPOUND_HEAD
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
+16 - COMPOUND_TAIL
+ A compound page tail (see description above).
+17 - HUGE
this is an integral part of a HugeTLB page
-
-19. HWPOISON
+19 - HWPOISON
hardware detected memory corruption on this page: don't touch the data!
-
-20. NOPAGE
+20 - NOPAGE
no page frame exists at the requested address
-
-21. KSM
+21 - KSM
identical memory pages dynamically shared between one or more processes
-
-22. THP
+22 - THP
contiguous pages which construct transparent hugepages
-
-23. BALLOON
+23 - BALLOON
balloon compaction page
-
-24. ZERO_PAGE
+24 - ZERO_PAGE
zero page for pfn_zero or huge_zero page
-
-25. IDLE
+25 - IDLE
page has not been accessed since it was marked idle (see
Documentation/vm/idle_page_tracking.txt). Note that this flag may be
stale in case the page was accessed via a PTE. To make sure the flag
- is up-to-date one has to read /sys/kernel/mm/page_idle/bitmap first.
-
- [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
+ is up-to-date one has to read ``/sys/kernel/mm/page_idle/bitmap`` first.
+
+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 the tools/vm directory can be used to query the
above flags.
-Using pagemap to do something useful:
+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
+ 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.
+ 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.
+ 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,
@@ -171,7 +184,8 @@ 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:
+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
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> --- Documentation/vm/pagemap.txt | 164 +++++++++++++++++++++++-------------------- 1 file changed, 89 insertions(+), 75 deletions(-)