Re: [PATCH v10 2/6] userfaultfd: update documentation to describe UFFD_FEATURE_WP_ASYNC

[Peter Xu]

On Thu, Feb 02, 2023 at 04:29:11PM +0500, Muhammad Usama Anjum wrote:
> Explain the difference created by UFFD_FEATURE_WP_ASYNC to the write
> protection (UFFDIO_WRITEPROTECT_MODE_WP) mode.
> 
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> ---
>  Documentation/admin-guide/mm/userfaultfd.rst | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
> index 83f31919ebb3..4747e7bd5b26 100644
> --- a/Documentation/admin-guide/mm/userfaultfd.rst
> +++ b/Documentation/admin-guide/mm/userfaultfd.rst
> @@ -221,6 +221,13 @@ former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter
>  you still need to supply a page when ``UFFDIO_REGISTER_MODE_MISSING`` was
>  used.
>  
> +If ``UFFD_FEATURE_WP_ASYNC`` is set while calling ``UFFDIO_API`` ioctl, the
> +behaviour of ``UFFDIO_WRITEPROTECT_MODE_WP`` changes such that faults for

UFFDIO_WRITEPROTECT_MODE_WP is only a flag in UFFDIO_WRITEPROTECT, while
it's forbidden only when not specified.

> +anon and shmem are resolved automatically by the kernel instead of sending
> +the message to the userfaultfd. The hugetlb isn't supported. The ``pagemap``
> +file can be read to find which pages have ``PM_UFFD_WP`` flag set which
> +means they are write-protected.

Here's my version. Please feel free to do modifications on top.

  If the userfaultfd context (that has ``UFFDIO_REGISTER_MODE_WP``
  registered against) has ``UFFD_FEATURE_WP_ASYNC`` feature enabled, it
  will work in async write protection mode.  It can be seen as a more
  accurate version of soft-dirty tracking, meanwhile the results will not
  be easily affected by other operations like vma merging.

  Comparing to the generic mode, the async mode will not generate any
  userfaultfd message when the protected memory range is written.  Instead,
  the kernel will automatically resolve the page fault immediately by
  dropping the uffd-wp bit in the pgtables.  The user app can collect the
  "written/dirty" status by looking up the uffd-wp bit for the pages being
  interested in /proc/pagemap.

  The page will be under track of uffd-wp async mode until the page is
  explicitly write-protected by ``UFFDIO_WRITEPROTECT`` ioctl with the mode
  flag ``UFFDIO_WRITEPROTECT_MODE_WP`` set.  Trying to resolve a page fault
  that was tracked by async mode userfaultfd-wp is invalid.

  Currently ``UFFD_FEATURE_WP_ASYNC`` only support anonymous and shmem.
  Hugetlb is not yet supported.

-- 
Peter Xu