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

[Muhammad Usama Anjum]

Hi Randy,

Thank you for your review.

On 6/14/23 9:20 AM, Randy Dunlap wrote:
> 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@xxxxxxxxxxxxx>
>> ---
>> 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
I'll make updates on all the places you have suggested.

> 
>> +  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
> 
> 

-- 
BR,
Muhammad Usama Anjum