[PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Hello,

This patch series implements a new ioctl on the pagemap proc fs file to
get, clear and perform both get and clear at the same time atomically on
the specified range of the memory.

Soft-dirty PTE bit of the memory pages can be viewed by using pagemap
procfs file. The soft-dirty PTE bit for the whole memory range of the
process can be cleared by writing to the clear_refs file. This series
adds features that weren't present earlier.
- There is no atomic get soft-dirty PTE bit status and clear operation
  present.
- The soft-dirty PTE bit of only a part of memory cannot be cleared.

Historically, soft-dirty PTE bit tracking has been used in the CRIU
project. The proc fs interface is enough for that as I think the process
is frozen. We have the use case where we need to track the soft-dirty
PTE bit for the running processes. We need this tracking and clear
mechanism of a region of memory while the process is running to emulate
the getWriteWatch() syscall of Windows. This syscall is used by games to
keep track of dirty pages and keep processing only the dirty pages. This
new ioctl can be used by the CRIU project and other applications which
require soft-dirty PTE bit information.

As in the current kernel there is no way to clear a part of memory (instead
of clearing the Soft-Dirty bits for the entire process) and get+clear
operation cannot be performed atomically, there are other methods to mimic
this information entirely in userspace with poor performance:
- The mprotect syscall and SIGSEGV handler for bookkeeping
- The userfaultfd syscall with the handler for bookkeeping
Some benchmarks can be seen [1].

This ioctl can be used by the CRIU project and other applications which
require soft-dirty PTE bit information. The following operations are
supported in this ioctl:
- Get the pages that are soft-dirty.
- Clear the pages which are soft-dirty.
- The optional flag to ignore the VM_SOFTDIRTY and only track per page
soft-dirty PTE bit

There are two decisions which have been taken about how to get the output
from the syscall.
- Return offsets of the pages from the start in the vec
- Stop execution when vec is filled with dirty pages
These two arguments doesn't follow the mincore() philosophy where the
output array corresponds to the address range in one to one fashion, hence
the output buffer length isn't passed and only a flag is set if the page
is present. This makes mincore() easy to use with less control. We are
passing the size of the output array and putting return data consecutively
which is offset of dirty pages from the start. The user can convert these
offsets back into the dirty page addresses easily. Suppose, the user want
to get first 10 dirty pages from a total memory of 100 pages. He'll
allocate output buffer of size 10 and the ioctl will abort after finding the
10 pages. This behaviour is needed to support Windows' getWriteWatch(). The
behaviour like mincore() can be achieved by passing output buffer of 100
size. This interface can be used for any desired behaviour.

[1] https://lore.kernel.org/lkml/54d4c322-cd6e-eefd-b161-2af2b56aae24@collabora.com/

Regards,
Muhammad Usama Anjum

Muhammad Usama Anjum (4):
  fs/proc/task_mmu: update functions to clear the soft-dirty PTE bit
  fs/proc/task_mmu: Implement IOCTL to get and clear soft dirty PTE bit
  selftests: vm: add pagemap ioctl tests
  mm: add documentation of the new ioctl on pagemap

 Documentation/admin-guide/mm/soft-dirty.rst |  42 +-
 fs/proc/task_mmu.c                          | 342 ++++++++++-
 include/uapi/linux/fs.h                     |  23 +
 tools/include/uapi/linux/fs.h               |  23 +
 tools/testing/selftests/vm/.gitignore       |   1 +
 tools/testing/selftests/vm/Makefile         |   2 +
 tools/testing/selftests/vm/pagemap_ioctl.c  | 649 ++++++++++++++++++++
 7 files changed, 1050 insertions(+), 32 deletions(-)
 create mode 100644 tools/testing/selftests/vm/pagemap_ioctl.c

-- 
2.30.2