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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 v20 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Wed, 21 Jun 2023 12:24:03 +0500 Message-Id: <20230621072404.2918101-5-usama.anjum@collabora.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230621072404.2918101-1-usama.anjum@collabora.com> References: <20230621072404.2918101-1-usama.anjum@collabora.com> MIME-Version: 1.0 X-Stat-Signature: 7193pyhh5j1mt9cbkhhoye1kpzcbr78k X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: ED040C0022 X-Rspam-User: X-HE-Tag: 1687332295-921843 X-HE-Meta: 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 5U0Rw/6E EFtG2K8abVyca7uQEP/gKAFyfJ6XpzVdP6HOtk6Iy59IxZZ/d8Pum3WNs+Bu5GPUXpx1SyyRys4QTXCZMfvgPitdGF2I7kC5Zo/nkBw3KfnDd+SEad3xMQ0AqJvbityGjkxgmPnyq1v+TCqUjMcNIKLh+E/mRfwlwQiXLGJFihQw3zvH0nUVnv91LqLCiBq0msToVuVBNCV4uDL7wHzPmzy28OKzdPYDMz0bAgpVrnD/Ekpa5+3QpzdTKg6riqKTqmKZlrhxEsblfp7IhOVGd1AyNo/OW2Mrj24MoB5x71PcAI+4= 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 v20: - Add new flag in documentation Changes in v19: - Improved documentatiom with punctuation marks 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..84c14ce86ad4 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``), swapped + (``PAGE_IS_SWAPPED``) or page has pfn zero (``PAGE_IS_PFNZERO``). +- 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`` unset, ``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 housekeeping 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 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.