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 (fromfs/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 Soft-Dirty PTEs)
Bit 56 page exclusively mapped (since 4.2)
Bit 57 pte is uffd-wp write-protected (since 5.13) (see 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/mm 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):
LOCKED
ERROR
REFERENCED
UPTODATE
DIRTY
LRU
ACTIVE
SLAB
WRITEBACK
RECLAIM
BUDDY
MMAP
ANON
SWAPCACHE
SWAPBACKED
COMPOUND_HEAD
COMPOUND_TAIL
HUGE
UNEVICTABLE
HWPOISON
NOPAGE
KSM
THP
OFFLINE
ZERO_PAGE
IDLE
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/SLUB kernel memory allocator. When compound page is used, either will only set this flag on the head page.
- 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 (HugeTLB Pages), 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 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.
Using pagemap to do something useful¶
The general procedure for using pagemap to find out about a process’ memory usage goes like this:
Read
/proc/pid/maps
to determine which parts of the memory space are mapped to what.Select the maps you are interested in -- all of them, or a particular library, or the stack or the heap, etc.
Open
/proc/pid/pagemap
and seek to the pages you would like to examine.Read a u64 for each page from pagemap.
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.
Pagemap Scan IOCTL¶
The PAGEMAP_SCAN
IOCTL on the pagemap file can be used to get or optionally
clear the info about page table entries. The following operations are supported
in this IOCTL:
Scan the address range and get the memory ranges matching the provided criteria. This is performed when the output buffer is specified.
Write-protect the pages. The
PM_SCAN_WP_MATCHING
is used to write-protect the pages of interest. ThePM_SCAN_CHECK_WPASYNC
aborts the operation if non-Async Write Protected pages are found. ThePM_SCAN_WP_MATCHING
can be used with or withoutPM_SCAN_CHECK_WPASYNC
.Both of those operations can be combined into one atomic operation where we can get and write protect the pages as well.
Following flags about pages are currently supported:
PAGE_IS_WPALLOWED
- Page has async-write-protection enabledPAGE_IS_WRITTEN
- Page has been written to from the time it was write protectedPAGE_IS_FILE
- Page is file backedPAGE_IS_PRESENT
- Page is present in the memoryPAGE_IS_SWAPPED
- Page is in swappedPAGE_IS_PFNZERO
- Page has zero PFNPAGE_IS_HUGE
- Page is THP or Hugetlb backedPAGE_IS_SOFT_DIRTY
- Page is soft-dirty
The struct pm_scan_arg
is used as the argument of the IOCTL.
The size of the
struct pm_scan_arg
must be specified in thesize
field. This field will be helpful in recognizing the structure if extensions are done later.The flags can be specified in the
flags
field. ThePM_SCAN_WP_MATCHING
andPM_SCAN_CHECK_WPASYNC
are the only added flags at this time. The get operation is optionally performed depending upon if the output buffer is provided or not.The range is specified through
start
andend
.The walk can abort before visiting the complete range such as the user buffer can get full etc. The walk ending address is specified in``end_walk``.
The output buffer of
struct page_region
array and size is specified invec
andvec_len
.The optional maximum requested pages are specified in the
max_pages
.The masks are specified in
category_mask
,category_anyof_mask
,category_inverted
andreturn_mask
.
Find pages which have been written and WP them as well:
struct pm_scan_arg arg = {
.size = sizeof(arg),
.flags = PM_SCAN_CHECK_WPASYNC | PM_SCAN_CHECK_WPASYNC,
..
.category_mask = PAGE_IS_WRITTEN,
.return_mask = PAGE_IS_WRITTEN,
};
Find pages which have been written, are file backed, not swapped and either present or huge:
struct pm_scan_arg arg = {
.size = sizeof(arg),
.flags = 0,
..
.category_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED,
.category_inverted = PAGE_IS_SWAPPED,
.category_anyof_mask = PAGE_IS_PRESENT | PAGE_IS_HUGE,
.return_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED |
PAGE_IS_PRESENT | PAGE_IS_HUGE,
};
The PAGE_IS_WRITTEN
flag can be considered as a better-performing alternative
of soft-dirty flag. It doesn’t get affected by VMA merging of the kernel and hence
the user can find the true soft-dirty pages in case of normal pages. (There may
still be extra dirty pages reported for THP or Hugetlb pages.)
“PAGE_IS_WRITTEN” category is used with uffd write protect-enabled ranges to implement memory dirty tracking in userspace:
The userfaultfd file descriptor is created with
userfaultfd
syscall.The
UFFD_FEATURE_WP_UNPOPULATED
andUFFD_FEATURE_WP_ASYNC
features are set byUFFDIO_API
IOCTL.The memory range is registered with
UFFDIO_REGISTER_MODE_WP
mode throughUFFDIO_REGISTER
IOCTL.Then any part of the registered memory or the whole memory region must be write protected using
PAGEMAP_SCAN
IOCTL with flagPM_SCAN_WP_MATCHING
or theUFFDIO_WRITEPROTECT
IOCTL can be used. Both of these perform the same operation. The former is better in terms of performance.Now the
PAGEMAP_SCAN
IOCTL can be used to either just find pages which have been written to since they were last marked and/or optionally write protect the pages as well.