Re: [PATCH v7 0/4] Implement IOCTL to get and/or the clear info about PTEs

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

On 1/9/23 11:45 AM, Muhammad Usama Anjum wrote:
> *Changes in v7:*
> - Add uffd wp async
> - Update the IOCTL to use uffd under the hood instead of soft-dirty
>   flags
> 
> Stop using the soft-dirty flags for finding which pages have been
> written to. It is too delicate and wrong as it shows more soft-dirty
> pages than the actual soft-dirty pages. There is no interest in
> correcting it [A][B] as this is how the feature was written years ago.
> It shouldn't be updated to changed behaviour. Peter Xu has suggested
> using the async version of the UFFD WP [C] as it is based inherently
> on the PTEs.
> 
> So in this patch series, I've added a new mode to the UFFD which is
> asynchronous version of the write protect. When this variant of the
> UFFD WP is used, the page faults are resolved automatically by the
> kernel. The pages which have been written-to can be found by reading
> pagemap file (!PM_UFFD_WP). This feature can be used successfully to
> find which pages have been written to from the time the pages were
> write protected. This works just like the soft-dirty flag without
> showing any extra pages which aren't soft-dirty in reality.
Any thoughts on this version are highly welcome. Please review.

> 
> [A] https://lore.kernel.org/all/20221220162606.1595355-1-usama.anjum@collabora.com
> [B] https://lore.kernel.org/all/20221122115007.2787017-1-usama.anjum@collabora.com
> [C] https://lore.kernel.org/all/Y6Hc2d+7eTKs7AiH@x1n
> 
> *Changes in v6:*
> - Updated the interface and made cosmetic changes
> 
> *Cover Letter in v5:*
> Hello,
> 
> This patch series implements IOCTL on the pagemap procfs file to get the
> information about the page table entries (PTEs). The following operations
> are supported in this ioctl:
> - Get the information if the pages are soft-dirty, file mapped, present
>   or swapped.
> - Clear the soft-dirty PTE bit of the pages.
> - Get and clear the soft-dirty PTE bit of the pages atomically.
> 
> Soft-dirty PTE bit of the memory pages can be read by using the 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. 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 here[1]. This series adds features that weren't
> present earlier:
> - 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 procfs interface is enough for finding the soft-dirty bit
> status and clearing the soft-dirty bit of all the pages of a process.
> We have the use case where we need to track the soft-dirty PTE bit for
> only specific pages on demand. 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 to process only the dirty pages.
> 
> The information related to pages if the page is file mapped, present and
> swapped is required for the CRIU project[2][3]. The addition of the
> required mask, any mask, excluded mask and return masks are also required
> for the CRIU project[2].
> 
> The IOCTL returns the addresses of the pages which match the specific masks.
> The page addresses are returned in struct page_region in a compact form.
> The max_pages is needed to support a use case where user only wants to get
> a specific number of pages. So there is no need to find all the pages of
> interest in the range when max_pages is specified. The IOCTL returns when
> the maximum number of the pages are found. The max_pages is optional. If
> max_pages is specified, it must be equal or greater than the vec_size.
> This restriction is needed to handle worse case when one page_region only
> contains info of one page and it cannot be compacted. This is needed to
> emulate the Windows getWriteWatch() syscall.
> 
> Some non-dirty pages get marked as dirty because of the kernel's
> internal activity (such as VMA merging as soft-dirty bit difference isn't
> considered while deciding to merge VMAs). The dirty bit of the pages is
> stored in the VMA flags and in the per page flags. If any of these two bits
> are set, the page is considered to be soft dirty. Suppose you have cleared
> the soft dirty bit of half of VMA which will be done by splitting the VMA
> and clearing soft dirty bit flag in the half VMA and the pages in it. Now
> kernel may decide to merge the VMAs again. So the half VMA becomes dirty
> again. This splitting/merging costs performance. The application receives
> a lot of pages which aren't dirty in reality but marked as dirty.
> Performance is lost again here. Also sometimes user doesn't want the newly
> allocated memory to be marked as dirty. PAGEMAP_NO_REUSED_REGIONS flag
> solves both the problems. It is used to not depend on the soft dirty flag
> in the VMA flags. So VMA splitting and merging doesn't happen. It only
> depends on the soft dirty bit of the individual pages. Thus by using this
> flag, there may be a scenerio such that the new memory regions which are
> just created, doesn't look dirty when seen with the IOCTL, but look dirty
> when seen from procfs. This seems okay as the user of this flag know the
> implication of using it.
> 
> [1] https://lore.kernel.org/lkml/54d4c322-cd6e-eefd-b161-2af2b56aae24@collabora.com/
> [2] https://lore.kernel.org/all/YyiDg79flhWoMDZB@gmail.com/
> [3] https://lore.kernel.org/all/20221014134802.1361436-1-mdanylo@google.com/
> 
> Regards,
> Muhammad Usama Anjum
> 
> Muhammad Usama Anjum (4):
>   userfaultfd: Add UFFD WP Async support
>   userfaultfd: split mwriteprotect_range()
>   fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about
>     PTEs
>   selftests: vm: add pagemap ioctl tests
> 
>  fs/proc/task_mmu.c                         | 300 +++++++
>  fs/userfaultfd.c                           | 161 ++--
>  include/linux/userfaultfd_k.h              |  10 +
>  include/uapi/linux/fs.h                    |  50 ++
>  include/uapi/linux/userfaultfd.h           |   6 +
>  mm/userfaultfd.c                           |  40 +-
>  tools/include/uapi/linux/fs.h              |  50 ++
>  tools/testing/selftests/vm/.gitignore      |   1 +
>  tools/testing/selftests/vm/Makefile        |   5 +-
>  tools/testing/selftests/vm/pagemap_ioctl.c | 884 +++++++++++++++++++++
>  10 files changed, 1424 insertions(+), 83 deletions(-)
>  create mode 100644 tools/testing/selftests/vm/pagemap_ioctl.c
> 

-- 
BR,
Muhammad Usama Anjum