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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 v25 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Thu, 13 Jul 2023 15:14:14 +0500 Message-Id: <20230713101415.108875-5-usama.anjum@collabora.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230713101415.108875-1-usama.anjum@collabora.com> References: <20230713101415.108875-1-usama.anjum@collabora.com> MIME-Version: 1.0 X-Rspamd-Queue-Id: 43CE280004 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: c4ua9xporfjg4ahxirr9s9w9m5h5meyx X-HE-Tag: 1689243302-910864 X-HE-Meta: U2FsdGVkX19UN1hy2aEFgw5WNEW1KQ25jb5iDynwJYMfbJ629G4cqC4g84SyguODHzFvCWq5dA0/Bflv1kVBs0mWSw3HQ0RQY8/RyOl0y2LGf1PfH+2XD7fqvExbAkxF2AYFUkUp9EjNI7lyytqZNNVR0NEB515b/9lzGjKagbFDZqOwvPLixPVlp6YLz11REh5Yx5qSga+9TCzHh5fIwEuOEIBUT8H7LIAe6zM4b9v4d4UC/5JrF1YjmZ+21jrhqDnraR8oZSyZG5zGyCvqhHPNj5DrFQOu0OheJUzORWAwB7pxxRy1Jmti0RJnHi/qZzNfEVEOrvuO5cJIPGGkfQlU6zCOyJUcEZ7FlzegkVU65LRZuqE++qKS0PSsq7SLxj8fAC0g3eQ8UFE70GKmEVVbRa4Zg8hkvaGwu+tnZhjb/jswjEmqV3MGr5++HKNamkn7OTQmHZTIM+zdgn+Y/vFHrsOg8YpQhXkbuQ4S540tJA76qaQYyqK2rWv+dwlo6YOS3zvXloqGrHQRrSe2nVBY6KARABbaXz1GPxSL9zebwXk0qp20AHPgn8remZtsr7rDqiap/taMZFP+SOUbpuDzpW/QYmvHxCzZYKLG/0flvoVcBw0YSI2kbki9MAsmaLa/8fkmVapGUbCA9PyiiV0FQd62BRphDGsgYPYAtOqQxIBaerUhHi82VminMRUivpQ0vDPMeuDiej6nQ4PDVsmb/xKA2VRdRGVAd3pV0Wmujf3XVULBpALtm/+hn3L+xrIt7B6UOi7GN6FMEBWsb0pAw2zw/9YwejQ/dMM8yF27VW0AzbM5ENonTranwxdaNv+rd5Dvotw6tuEjEP8PqvDZ+68KUKQiU7pzZPbab/AVh50zJ0xUJE9uRQ+bkkOiAGLYpINqLDErDQRlQ/NdwHLfgu42lqme33OMUEvc/95SRoktDrwDuKK/82xwvOXlvA13SHSXrCgP9VEmOSa kE1Ugs02 Ek58WDG+n6gddHoG8UsSzL9n7nAcE7KMidJak2ObL4wxZh+vLaIXZgx2DparupqBLuACS1bwJ0oQuADKD0lFjWOCs0KEAx9R6mEC1EHJiOEuPtNYOXjeSbLfjNjyw2zSWUzwBKfCaQDB4IHeQtPMBHw9ngpYG8o2wsNht/WmsGfgnTNGIRtdOMkquRHFgadKDsTCTDVxGaHIJQod+wERVLyn3YDCRtdhc2dCFqd4DkayGrx9KhIjZj1Y7hB1x4pKA1Vba+TMhyD2NGduq+nAEh+2Y5qdcovleyHANqYoOjZYkJc+pbD8Taw/DCW4wH8AuJx1hCYpncFTX1UmjXGw/6IiJTTg+IREXLlQnBH8DIQqlfyhjsjwf4Cq0kMbO6SDlZos0 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 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 rst --- 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..4bdad8eff009 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 ``end``. + 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.