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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 v29 5/6] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Fri, 11 Aug 2023 23:08:41 +0500 Message-Id: <20230811180842.3141781-6-usama.anjum@collabora.com> X-Mailer: git-send-email 2.40.1 In-Reply-To: <20230811180842.3141781-1-usama.anjum@collabora.com> References: <20230811180842.3141781-1-usama.anjum@collabora.com> MIME-Version: 1.0 X-Stat-Signature: oa94xkizrdjxqxnu3o3bay9tbz9f65uh X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 4D54E8001F X-Rspam-User: X-HE-Tag: 1691777369-490322 X-HE-Meta: U2FsdGVkX1+3BRHpAY0WOCSlXzjOxo3SnkMi+iOnA8Ba+PU1Hcon5Jv7J2aw2bbg82WghCnxpk64qZPN1+4pnQN1YIDHiqRk4XdBfDbl1SBR+YQj8mSG4/I0cf9CUb5CV1hMO+cjch9Tk/DLMhcoZOJdYH5sAwerghSqyVGWjATAWih+PY0bvequgZmTzATo6/coIHgifUPpAGmhuq0ZO73S6FWdki2H4wRZMuoW9A2ss7i5W3RDkPVo+ADY00GtS6dPs+62BIkN/yLjaXAuOTfzTKzGmiJx/aX1sOMLKDY6cSivLgzr7enjkGrun8E6RanU/xoy1TNs1QsQEDEb0gyA/mMo5VRhfeSgS+XVqd+wbHOpD6D1jf4PN0W+EygfPuTTkqRDnaGfszST6gbLRGIviSVaqbmWpv3dTbtjWX+kcYUigJChrazYUluUjt5jLqG5HbKEZxdHAMXZAmyd01zRp9tzNsM27oMsHWVD3lqtcA6ARGSyBR1semBOBw6+ysG+lj0Ug7MQiZF0eGfVzOxZ2HLZj5XBpaEO1C+NLCZOeh5PARtZ4bOepybs6qxmnXqGKoKeIGz4cT6E6Qw2/1YvlH9vddS7iy8B895jbhEhpXWWW/MORJYPi4QwYf3kkL4TtcxwJSf9OtbltIuBl4A44KMFRrogmoTxSG2lxrBWHD5EFYle/25OFlyZk6swtEaLKdAVM3BaBjXX3kHVk0dcLTaxxpV7uWRDDp/ijUEypij/z6VCF5wpfFsuZoXAYQNtD6pJG295Orq3oj6dGnWTOCFMi5Grpz7UbKqzfsIInmSpn+aEhjxfJQdrtiz6ZyTUW3aDRnYCKWqMnC7MdfQxKCKfFC8ZprLw468mOPm2dQvCaE6DSEYlnaaFaGGyYuTxaflCrRiWXuNAT4NuoKHcTl69oRUsQluj55NnWOfBqAfO96JizoD7SwIr+F1uj6azCf1K0dPYtBWgXG+ LIuwRwSp o4ZnwSDci48xffBh1JqYsLqqXJJadpb3PcIyTeqw4W+GD0rP640RNObIb1E1j1CZmKeYofqCHwPDrnzOwfouiPd0Ac0IBYlguk8n84f7Q2sb0xfnhfmUKdBMc7iHpNKsNbiFpE/vLOo9FvqtOoEuOiuBzitBzxrA2IZhoviPXvdey+5DgC42/yEdpOntvNcmOvTd6N7zMT5em2QMwmfiw0vFBMJvtIYIWMvn8ZucW8U4R8JC1HTkQF3/lyLabMLY5HTd5AkQcw49eABizW/vVifHXEbyvRfbJjGooTXbw7xd3tic= 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 v29: - Revamp the documentation and make it better Changes in v26: - Update documentation according to latest revision Changes in v21: - Update according to changed interface 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 | 89 ++++++++++++++++++++++++ 1 file changed, 89 insertions(+) diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst index c8f380271cad8..b5ccdad6103ed 100644 --- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -227,3 +227,92 @@ 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: + +- Scan the address range and get the memory ranges matching the provided criteria. + This is performed by default when the output buffer is specified. +- Write-protect the pages. The ``PM_SCAN_WP_MATCHING`` is used to write-protect + the pages of interest. The ``PM_SCAN_CHECK_WPASYNC`` aborts the operation if + non-Async Write Protected pages are found. The ``PM_SCAN_WP_MATCHING`` can be + used with or without ``PM_SCAN_CHECK_WPASYNC``. +- Both of those operations can be combined into one atomic operation where we can + get and write protect the pages as well. + +Following flags about pages are currently supported: + +- ``PAGE_IS_WPALLOWED`` - Page has async-write-protection enabled +- ``PAGE_IS_WRITTEN`` - Page has been written to from the time it was write protected +- ``PAGE_IS_FILE`` - Page is file backed +- ``PAGE_IS_PRESENT`` - Page is present in the memory +- ``PAGE_IS_SWAPPED`` - Page is in swapped +- ``PAGE_IS_PFNZERO`` - Page has zero PFN +- ``PAGE_IS_HUGE`` - Page is THP or Hugetlb backed + +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_WP_MATCHING`` + and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get + operation is optionally performed depending upon if the output buffer is + provided or not. + 3. The range is specified through ``start`` and ``end``. + 4. The walk can abort before visiting the complete range such as the user buffer + can get full etc. The walk ending address is specified in``end_walk``. + 5. The output buffer of ``struct page_region`` array and size is specified in + ``vec`` and ``vec_len``. + 6. The optional maximum requested pages are specified in the ``max_pages``. + 7. The masks are specified in ``category_mask``, ``category_anyof_mask``, + ``category_inverted`` and ``return_mask``. + +Find pages which have been written and WP them as well:: + + struct pm_scan_arg arg = { + .size = sizeof(arg), + .flags = PM_SCAN_CHECK_WPASYNC | PM_SCAN_CHECK_WPASYNC, + .. + .category_mask = PAGE_IS_WRITTEN, + .return_mask = PAGE_IS_WRITTEN, + }; + +Find pages which have been written, are file backed, not swapped and either +present or huge:: + + struct pm_scan_arg arg = { + .size = sizeof(arg), + .flags = 0, + .. + .category_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED, + .category_inverted = PAGE_IS_SWAPPED, + .category_anyof_mask = PAGE_IS_PRESENT | PAGE_IS_HUGE, + .return_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED | + PAGE_IS_PRESENT | PAGE_IS_HUGE, + }; + +The ``PAGE_IS_WRITTEN`` flag can be considered as a better-performing alternative +of soft-dirty flag. It doesn't get affected by VMA merging of the kernel and hence +the user can find the true soft-dirty pages in case of normal pages. (There may +still be extra dirty pages reported for THP or Hugetlb pages.) + +"PAGE_IS_WRITTEN" category is used with uffd write protect-enabled ranges to +implement memory dirty tracking in userspace: + + 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_WP_MATCHING`` + 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 since they were last marked and/or optionally write protect + the pages as well.