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Howlett" , Yun Zhou , Suren Baghdasaryan , Alex Sierra , Muhammad Usama Anjum , Matthew Wilcox , Pasha Tatashin , Axel Rasmussen , "Gustavo A . R . Silva" , Dan Williams , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Greg KH , kernel@collabora.com Subject: [PATCH v14 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Tue, 18 Apr 2023 11:20:07 +0500 Message-Id: <20230418062008.1434826-5-usama.anjum@collabora.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230418062008.1434826-1-usama.anjum@collabora.com> References: <20230418062008.1434826-1-usama.anjum@collabora.com> MIME-Version: 1.0 X-Rspamd-Queue-Id: D7F0314000D X-Stat-Signature: tgbeuiemrnrfbfdhdwmqu55hmsqc4ina X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1681798859-278124 X-HE-Meta: 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 rk9m78ye ixq3pvvndx6okID5r6j8hDX1lTbmQRQDgLmGpXI5vAMv3F2SQQLSuzMeqK5ahtE88RxkOx+SV23zjSBppCz+co8jqbij1CNMo6VUgClugRb0nYPcI7C1K/B2et8jrTmDIRcmNYGFuqpgPBlhhgQhRRfo/zYg+UOR7Pk5KwGzT/OfDmYJd2YLw8vBm0/SMAmfFY2QRoOb8O3d14NMfyhcdfvsrnjA9fc3YBMjiNeDPDBb3Swd3amGIkm/sZTrTPyR153MWC8S5mTiSttdTm+fMunxdj4svWxwKKccajlgJs0XLkEwvD6oRmdHn7lvhjtYqFl58 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Add some explanation and method to use write-protection and written-to on memory range. Signed-off-by: Muhammad Usama Anjum --- Changes in v11: - Add more documentation --- Documentation/admin-guide/mm/pagemap.rst | 56 ++++++++++++++++++++++++ 1 file changed, 56 insertions(+) diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst index c8f380271cad..a3e08f15b433 100644 --- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -227,3 +227,59 @@ 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``), + file mapped (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``) or swapped + (``PAGE_IS_SWAPPED``). +- Find pages which have been written-to and write protect the pages atomically + (atomic ``PAGE_IS_WRITTEN + PAGEMAP_WP_ENGAGE``) + +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 flag 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`` + 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 + ``PAGEMAP_WP_ENGAGE`` 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 +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 +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 + be write protected using the ``UFFDIO_WRITEPROTECT`` IOCTL. + 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which + have been written-to or optionally write protect the pages as well.