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

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

On 2/10/23 12:26 AM, Peter Xu wrote:
> 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@collabora.com>
>> ---
>>  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)
I'll update the documentation.

> 
>> +
>> +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,
> 

-- 
BR,
Muhammad Usama Anjum