docs/admin-guide/mm: start moving here files from Documentation/vm

Several documents in Documentation/vm fit quite well into the "admin/user
guide" category. The documents that don't overload the reader with lots of
implementation details and provide coherent description of certain feature
can be moved to Documentation/admin-guide/mm.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 197 insertions, 0 deletions

diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
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+.. _pagemap:
+
+=============================
+Examining Process Page Tables
+=============================
+
+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 four 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/admin-guide/mm/soft-dirty.rst)
+    * Bit  56    page exclusively mapped (since 4.2)
+    * Bits 57-60 zero
+    * Bit  61    page is file-page or shared-anon (since 3.5)
+    * Bit  62    page swapped
+    * Bit  63    page present
+
+   Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs.
+   In 4.0 and 4.1 opens by unprivileged fail with -EPERM.  Starting from
+   4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN.
+   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.
+
+   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
+    17. HUGE
+    18. UNEVICTABLE
+    19. HWPOISON
+    20. NOPAGE
+    21. KSM
+    22. THP
+    23. BALLOON
+    24. ZERO_PAGE
+    25. IDLE
+
+ * ``/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, e.g. 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
+    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/admin-guide/mm/hugetlbpage.rst), the SLUB etc.
+    memory allocators and various device drivers. However in this interface,
+    only huge/giga pages are made visible to end users.
+16 - COMPOUND_TAIL
+    A compound page tail (see description above).
+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
+23 - BALLOON
+    balloon compaction page
+24 - ZERO_PAGE
+    zero page for pfn_zero or huge_zero page
+25 - IDLE
+    page has not been accessed since it was marked idle (see
+    Documentation/admin-guide/mm/idle_page_tracking.rst). 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
+   i.e. 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, e.g. 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, i.e. 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
+====================================
+
+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.
+
+Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
+always 12 at most architectures). Since Linux 3.11 their meaning changes
+after first clear of soft-dirty bits. Since Linux 4.2 they are used for
+flags unconditionally.