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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 v18 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Tue, 13 Jun 2023 15:29:04 +0500 Message-Id: <20230613102905.2808371-5-usama.anjum@collabora.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230613102905.2808371-1-usama.anjum@collabora.com> References: <20230613102905.2808371-1-usama.anjum@collabora.com> MIME-Version: 1.0 X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 37A5380005 X-Stat-Signature: i5n5ihk4m5xs9raorhuqbcnmtuyb94e7 X-HE-Tag: 1686652193-756861 X-HE-Meta: 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 4+o07kiB 3ficgostiTKGjuhy1nHntzQa+CMKUI1ReGvGJTH4/zwYXQfh+stobPmM76BSSkNLMOtkk9Od/fr9tW7aWXBpjwe/SmuxPcqvOoaoteMY/J0ojDglzc7gbJ7ydsGxIb31kNbOzKTiGezEdNHGnWSkldRa+h71+s2nQDsosrfCmf7vtz7FB7a9JKy/G0OUr8IekPFI61diX/YNdH0DuBFENvry2yshOaGsQATSpe+lXzlVSlfUQrARsI/jXQvOd74Dnd1+LNNm49lSWcJ2uVN5DviGhkrn87w7lNWlF5cMoMvGvXmw= 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 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``), + 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 + (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`` + 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 +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 ``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.