[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <8bf13c14-8f58-aa20-2c5e-d0a293f1686f@collabora.com>
Date: Wed, 14 Jun 2023 16:27:54 +0500
From: Muhammad Usama Anjum <usama.anjum@...labora.com>
To: Randy Dunlap <rdunlap@...radead.org>, Peter Xu <peterx@...hat.com>,
David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Michał Mirosław
<emmir@...gle.com>, Andrei Vagin <avagin@...il.com>,
Danylo Mocherniuk <mdanylo@...gle.com>,
Paul Gofman <pgofman@...eweavers.com>,
Cyrill Gorcunov <gorcunov@...il.com>,
Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>
Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Shuah Khan <shuah@...nel.org>,
Christian Brauner <brauner@...nel.org>,
Yang Shi <shy828301@...il.com>,
Vlastimil Babka <vbabka@...e.cz>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Yun Zhou <yun.zhou@...driver.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Alex Sierra <alex.sierra@....com>,
Matthew Wilcox <willy@...radead.org>,
Pasha Tatashin <pasha.tatashin@...een.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
"Gustavo A . R . Silva" <gustavoars@...nel.org>,
Dan Williams <dan.j.williams@...el.com>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v18 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN
IOCTL
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@...labora.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
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
Powered by blists - more mailing lists