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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 v19 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Thu, 15 Jun 2023 19:11:43 +0500 Message-Id: <20230615141144.665148-5-usama.anjum@collabora.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230615141144.665148-1-usama.anjum@collabora.com> References: <20230615141144.665148-1-usama.anjum@collabora.com> MIME-Version: 1.0 X-Stat-Signature: z3p7mpcr6zmiugf4fayihwbfjmnp7xua X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 0ADCDC006E X-Rspam-User: X-HE-Tag: 1686838362-547676 X-HE-Meta: U2FsdGVkX1/HeS45a40ytYrJ/ySdwN+UZg+BtvWk8ZFm2OP3DQRhcAjUNX7T+hgtJyqaNVrTsp8u/pqvMiNGV7TWQ7DUzjk/ZJfex57ceDIf8GD7r8B5lDDNrGE9uoSmN2QlZnPtswk2Td6frRZY0a8GCiEzwNOF6rLK5TVcj59+mPh4g5fJGM1lL9dUSUSq/LTZ+xfapATdoPPcEjyzh/wYWyUf47DqulL8tkRGy7DKqpHXhiiqSDrPDxS75d47Gvite7xwfTd1MHvSkHe0dlj2hP2oO795y+IM+BiR2Q6vWt8G5mrpvxPLGP2vC0IGbHl0v4STf6Z5HtcjlClQFCCQGcRWelQiHx4elFCbd9hFvKpVnYAQAHz/4VMmaFyP0NaElTzZxMYR1H7aYDuxdW6+0E6XIskdLxmBjqdxGzqdrqPWSanUCpl6iKUQ+d8MkJV96hbMSpqcjRj60WR1Y1v7DOI8czm4xyCpsoCsi4lQ2PR5SS5Vu6iuPXFdDeOk2RxN/pH8X+Sv7j+DDLe4XSYL3a1Ey+tpxrxI7YFSVa3zNZxUwBCWT9o9VLtedbuAFi7+20HN4oOrDb6fBOQED2F/LJC5lzjcb2CV5Ou3C+MAZJnVPXSHWMiUM6A8AYS+JqZ0H9oRsVbW5aE+xjoRzZy4ZwK+oPi1KSqZC/qJpaAFNhBuCMbQ/x/vLHTCfT5NCxMVaIUVJ8YQ+hbV0i10NhYIBCtz+bVE8Kol8HMNL4OxzWh1YiAbKtXzZSso+JnlON27N74a1BeS/KpyXMP9DIMMx2aMJXeeWOa4pLZc7lQtJXyQDFf7vhHWSpQ91Hbxa8fzo6j0T+5UceeEIwvJ4u6YtWZX758tRNwsF0x/GmtAC6IbOGW7XOScRLcPKyiLXGaOfvlH54xqPccOENV4bfhhhpNQ6uQhT8+IOBrGC3X1tbUl2dhVCt86L+ARpjEC9ajoJGfG9Kyc0gktIV4 w5vMGlL/ T8hu3+iiV5ToMpgWLXi4T0ayuOinjOtx95tXxGvMLG4hxw7i4b+u6dErcnQyUihgR+lDuCfjj2XvNQaH/6yceA0LIWJ5cmXv2izkqnPnGMIo7mbmJ5CYH77Kzog0JHTuLLFVadcxi9Yp6Vu3Lz6CaupVVYzeDIb0aKQkrxfFWj4rBUOGXAw/GcHTR8MIpDSkg3DxY7p29S61vnpcvJZvRs6buJ3cq8xR6R+43kTEiu9SYlVCVoT+eqcXd6Dj2Px6taJ7sEjUb1NJRwQT+Q944MmlWK8DPSrCyRAfqGDKSxjgT4Z0= 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 v19: - Improved documentatio 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..2e1c8344c04e 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`` 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.