Re: [PATCH 0/5] Add process_memwatch syscall

["Peter.Enderborg@sony.com" <Peter.Enderborg@sony.com>]

On 7/26/22 18:18, Muhammad Usama Anjum wrote:
> Hello,
> 
> This patch series implements a new syscall, process_memwatch. Currently,
> only the support to watch soft-dirty PTE bit is added. This syscall is
> generic to watch the memory of the process. There is enough room to add
> more operations like this to watch memory in the future.
> 
> Soft-dirty PTE bit of the memory pages can be viewed by using pagemap
> procfs file. The soft-dirty PTE bit for the memory in a process can be
> cleared by writing to the clear_refs file. This series adds features that
> weren't possible through the Proc FS interface.
> - There is no atomic get soft-dirty PTE bit status and clear operation
>   possible.
> - 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 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
> syscall 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 processi) 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
> 
>         long process_memwatch(int pidfd, unsigned long start, int len,
>                               unsigned int flags, void *vec, int vec_len);
> 
> This syscall can be used by the CRIU project and other applications which
> require soft-dirty PTE bit information. The following operations are
> supported in this syscall:
> - 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
> 

Why can it not be done as a IOCTL?


> 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 process_memwatch() syscall 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.
> 
> Regards,
> Muhammad Usama Anjum
> 
> Muhammad Usama Anjum (5):
>   fs/proc/task_mmu: make functions global to be used in other files
>   mm: Implement process_memwatch syscall
>   mm: wire up process_memwatch syscall for x86
>   selftests: vm: add process_memwatch syscall tests
>   mm: add process_memwatch syscall documentation
> 
>  Documentation/admin-guide/mm/soft-dirty.rst   |  48 +-
>  arch/x86/entry/syscalls/syscall_32.tbl        |   1 +
>  arch/x86/entry/syscalls/syscall_64.tbl        |   1 +
>  fs/proc/task_mmu.c                            |  84 +--
>  include/linux/mm_inline.h                     |  99 +++
>  include/linux/syscalls.h                      |   3 +-
>  include/uapi/asm-generic/unistd.h             |   5 +-
>  include/uapi/linux/memwatch.h                 |  12 +
>  kernel/sys_ni.c                               |   1 +
>  mm/Makefile                                   |   2 +-
>  mm/memwatch.c                                 | 285 ++++++++
>  tools/include/uapi/asm-generic/unistd.h       |   5 +-
>  .../arch/x86/entry/syscalls/syscall_64.tbl    |   1 +
>  tools/testing/selftests/vm/.gitignore         |   1 +
>  tools/testing/selftests/vm/Makefile           |   2 +
>  tools/testing/selftests/vm/memwatch_test.c    | 635 ++++++++++++++++++
>  16 files changed, 1098 insertions(+), 87 deletions(-)
>  create mode 100644 include/uapi/linux/memwatch.h
>  create mode 100644 mm/memwatch.c
>  create mode 100644 tools/testing/selftests/vm/memwatch_test.c
>