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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 v13 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Mon, 17 Apr 2023 17:56:29 +0500 Message-Id: <20230417125630.1146906-5-usama.anjum@collabora.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230417125630.1146906-1-usama.anjum@collabora.com> References: <20230417125630.1146906-1-usama.anjum@collabora.com> MIME-Version: 1.0 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 6AF8FC0005 X-Rspam-User: X-Stat-Signature: hg8qm68t9x981tzd8ib35mhk3yjrx69h X-HE-Tag: 1681736251-331934 X-HE-Meta: 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 eZJAlCwp xtyaZWNwzH3JI4ADCt/z1s40aU7GhU5UOxEumv/+Bk7uPilbv5E1N225lSfY2SHkvfrfRLAkwXwxBHwsIaL3Nt3BiqhkIl4l6g4x3+DvIjANiNHUNlA2ZEAowfDNFsaDKv14viq+ZTbM5AAcY17lvuWzZbPKomCny43NrTJcfMvTve+fd/PZtItwTqsfXkh5ZvnXe7ZtirbqBAuwSrGWAttZqVVW8C8Ils06gtH4K8CjpzFLIg9HZ7htrMJ2QvSzqOfA4O0wICPkjrOBiR6xa14Rz+kXkFhbMB8Km0PEzToGnWHA0EBq2mKaW49ymMA8wxri7 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.