Re: [PATCH v18 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL

[Randy Dunlap <rdunlap@infradead.org>]

Hi--

On 6/13/23 03:29, 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@collabora.com>
> ---
> Changes in v16:
> - Update the documentation
> 
> Changes in v11:
> - Add more documentation
> ---
>  Documentation/admin-guide/mm/pagemap.rst | 58 ++++++++++++++++++++++++
>  1 file changed, 58 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index c8f380271cad..3b977526d4b3 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -227,3 +227,61 @@ 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 or optionally
> +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``),

                                                written to

> +  file mapped (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``) or swapped
> +  (``PAGE_IS_SWAPPED``).
> +- Find pages which have been written-to and/or write protect the pages atomically

                                written to

> +  (atomic ``PM_SCAN_OP_GET + PM_SCAN_OP_WP``)
> +
> +The ``struct pm_scan_arg`` is used as the argument of the IOCTL.
> + 1. The size of the ``struct pm_scan_arg`` must be specified in the ``size``
> +    field. This field will be helpful in recognizing the structure if extensions
> +    are done later.
> + 2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_OP_GET``
> +    and ``PM_SCAN_OP_WP`` are the only added flags at this time.
> + 3. The range is specified through ``start`` and ``len``.
> + 4. The output buffer of ``struct page_region`` array and size is specified in
> +    ``vec`` and ``vec_len``.
> + 5. The optional maximum requested pages are specified in the ``max_pages``.
> + 6. The masks are specified in ``required_mask``, ``anyof_mask``,
> +    ``excluded_ mask`` and ``return_mask``.
> +    1.  To find if ``PAGE_IS_WRITTEN`` flag is set for pages which have
> +        ``PAGE_IS_FILE`` set and ``PAGE_IS_SWAPPED`` un-set, ``required_mask``

                                                        unset,

> +        is set to ``PAGE_IS_FILE``, ``exclude_mask`` is set to
> +        ``PAGE_IS_SWAPPED`` and ``return_mask`` is set to ``PAGE_IS_WRITTEN``.
> +        The output buffer in ``vec`` and length must be specified in ``vec_len``.
> +    2. To find pages which have either ``PAGE_IS_FILE`` or ``PAGE_IS_SWAPPED``
> +       set, ``anyof_masks`` is set to  ``PAGE_IS_FILE | PAGE_IS_SWAPPED``.
> +    3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is
> +       specified in ``required_mask`` and ``return_mask``. In addition to
> +       specifying the output buffer in ``vec`` and length in ``vec_len``, the
> +       ``PM_SCAN_OP_WP`` is specified in ``flags`` to perform write protect
> +       on the range as well.
> +
> +The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct
> +alternative of soft-dirty flag. It doesn't get affected by household chores (VMA

                                                              housekeeping ?

> +merging) of the kernel and hence the user can find the true soft-dirty pages
> +only. This IOCTL adds the atomic way to find which pages have been written and
> +write protect those pages again. This kind of operation is needed to efficiently
> +find out which pages have changed in the memory.
> +
> +To get information about which pages have been written-to or optionally write

                                                  written to

> +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_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features
> +    are set by ``UFFDIO_API`` IOCTL.
> + 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
> +    through ``UFFDIO_REGISTER`` IOCTL.
> + 4. Then the any part of the registered memory or the whole memory region must

            "the" or "any". Probably "any". Not both.

> +    be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_OP_WP``
> +    or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the
> +    same operation. The former is better in terms of performance.
> + 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which
> +    have been written-to and/or optionally write protect the pages as well.

                 written to


-- 
~Randy