Re: [PATCH v10 5/6] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL

[Peter Xu]

On Thu, Feb 02, 2023 at 04:29:14PM +0500, Muhammad Usama Anjum wrote:
> Add some explanation and method to use write-protection and written-to
> on memory range.
> 
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> ---
>  Documentation/admin-guide/mm/pagemap.rst | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index 6e2e416af783..1cb2189e9a0d 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -230,3 +230,27 @@ 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 and/or clear
> +the info about page table entries. The following operations are supported in
> +this IOCTL:
> +- Get the information if the pages have been written-to (``PAGE_IS_WRITTEN``),
> +  file mapped (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``) or swapped
> +  (``PAGE_IS_SWAPPED``).
> +- Write-protect the pages (``PAGEMAP_WP_ENGAGE``) to start finding which
> +  pages have been written-to.
> +- Find pages which have been written-to and write protect the pages
> +  (atomic ``PAGE_IS_WRITTEN + PAGEMAP_WP_ENGAGE``)

Could we extend this section a bit more?  Some points for reference:

  - The new struct you introduced, definitions of each of the fields, and
    generic use cases for each of the field/ops.

  - It'll be nice to list the OPs the new interface supports (GET,
    WP_ENGAGE, GET+WP_ENGAGE).

  - When should people use this rather than the old pagemap interface?
    What's the major problems to solve / what's the major difference?
    (Maybe nice to reference the Windows API too here)

> +
> +To get information about which pages have been written-to and/or write protect
> +the pages, following must be performed first in order:
> + 1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall.
> + 2. The ``UFFD_FEATURE_WP_ASYNC`` feature is set by ``UFFDIO_API`` IOCTL.
> + 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
> +    through ``UFFDIO_REGISTER`` IOCTL.
> +Then the any part of the registered memory or the whole memory region can be
> +write protected using the ``UFFDIO_WRITEPROTECT`` IOCTL or ``PAGEMAP_SCAN``
> +IOCTL.

This part looks good.

Thanks,

-- 
Peter Xu