mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-05 18:14:07 +08:00
dd21bfa425
Since bit 57 was exported for uffd-wp write-protected (commitfb8e37f35a
: "mm/pagemap: export uffd-wp protection information"), fixing it can reduce some unnecessary confusion. Link: https://lkml.kernel.org/r/20220301044538.3042713-1-yun.zhou@windriver.com Fixes:fb8e37f35a
("mm/pagemap: export uffd-wp protection information") Signed-off-by: Yun Zhou <yun.zhou@windriver.com> Reviewed-by: Peter Xu <peterx@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com> Cc: Florian Schmidt <florian.schmidt@nutanix.com> Cc: Ivan Teterevkov <ivan.teterevkov@nutanix.com> Cc: SeongJae Park <sj@kernel.org> Cc: Yang Shi <shy828301@gmail.com> Cc: David Hildenbrand <david@redhat.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: Miaohe Lin <linmiaohe@huawei.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Colin Cross <ccross@google.com> Cc: Alistair Popple <apopple@nvidia.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
233 lines
8.4 KiB
ReStructuredText
233 lines
8.4 KiB
ReStructuredText
.. _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
|
|
:ref:`Documentation/admin-guide/mm/soft-dirty.rst <soft_dirty>`)
|
|
* Bit 56 page exclusively mapped (since 4.2)
|
|
* Bit 57 pte is uffd-wp write-protected (since 5.13) (see
|
|
:ref:`Documentation/admin-guide/mm/userfaultfd.rst <userfaultfd>`)
|
|
* Bits 58-60 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.
|
|
|
|
The page-types tool in the tools/vm directory can be used to query the
|
|
number of times a page is mapped.
|
|
|
|
* ``/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. OFFLINE
|
|
24. ZERO_PAGE
|
|
25. IDLE
|
|
26. PGTABLE
|
|
|
|
* ``/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
|
|
The page is being locked for exclusive access, e.g. by undergoing read/write
|
|
IO.
|
|
7 - SLAB
|
|
The 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
|
|
(:ref:`Documentation/admin-guide/mm/hugetlbpage.rst <hugetlbpage>`),
|
|
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 - OFFLINE
|
|
The page is logically offline.
|
|
24 - ZERO_PAGE
|
|
Zero page for pfn_zero or huge_zero page.
|
|
25 - IDLE
|
|
The page has not been accessed since it was marked idle (see
|
|
:ref:`Documentation/admin-guide/mm/idle_page_tracking.rst <idle_page_tracking>`).
|
|
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.
|
|
26 - PGTABLE
|
|
The page is in use as a page table.
|
|
|
|
IO related page flags
|
|
---------------------
|
|
|
|
1 - ERROR
|
|
IO error occurred.
|
|
3 - UPTODATE
|
|
The page has up-to-date data.
|
|
ie. for file backed page: (in-memory data revision >= on-disk one)
|
|
4 - DIRTY
|
|
The page has been written to, hence contains new data.
|
|
i.e. for file backed page: (in-memory data revision > on-disk one)
|
|
8 - WRITEBACK
|
|
The page is being synced to disk.
|
|
|
|
LRU related page flags
|
|
----------------------
|
|
|
|
5 - LRU
|
|
The page is in one of the LRU lists.
|
|
6 - ACTIVE
|
|
The page is in the active LRU list.
|
|
18 - UNEVICTABLE
|
|
The 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
|
|
The page has been referenced since last LRU list enqueue/requeue.
|
|
9 - RECLAIM
|
|
The 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
|
|
The page is mapped to swap space, i.e. has an associated swap entry.
|
|
14 - SWAPBACKED
|
|
The 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.
|
|
|
|
Exceptions for Shared Memory
|
|
============================
|
|
|
|
Page table entries for shared pages are cleared when the pages are zapped or
|
|
swapped out. This makes swapped out pages indistinguishable from never-allocated
|
|
ones.
|
|
|
|
In kernel space, the swap location can still be retrieved from the page cache.
|
|
However, values stored only on the normal PTE get lost irretrievably when the
|
|
page is swapped out (i.e. SOFT_DIRTY).
|
|
|
|
In user space, whether the page is present, swapped or none can be deduced with
|
|
the help of lseek and/or mincore system calls.
|
|
|
|
lseek() can differentiate between accessed pages (present or swapped out) and
|
|
holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where
|
|
the pages are backed. For anonymous shared pages, the file can be found in
|
|
``/proc/pid/map_files/``.
|
|
|
|
mincore() can differentiate between pages in memory (present, including swap
|
|
cache) and out of memory (swapped out or none/non-allocated).
|
|
|
|
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.
|