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mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL
Add some explanation and method to use write-protection and written-to on memory range. Link: https://lkml.kernel.org/r/20230821141518.870589-6-usama.anjum@collabora.com Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com> Cc: Alex Sierra <alex.sierra@amd.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Andrei Vagin <avagin@gmail.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: Christian Brauner <brauner@kernel.org> Cc: Cyrill Gorcunov <gorcunov@gmail.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: David Hildenbrand <david@redhat.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Gustavo A. R. Silva <gustavoars@kernel.org> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Michal Miroslaw <emmir@google.com> Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl> Cc: Mike Rapoport (IBM) <rppt@kernel.org> Cc: Nadav Amit <namit@vmware.com> Cc: Pasha Tatashin <pasha.tatashin@soleen.com> Cc: Paul Gofman <pgofman@codeweavers.com> Cc: Peter Xu <peterx@redhat.com> Cc: Shuah Khan <shuah@kernel.org> Cc: Suren Baghdasaryan <surenb@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Yang Shi <shy828301@gmail.com> Cc: Yun Zhou <yun.zhou@windriver.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -227,3 +227,92 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
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always 12 at most architectures). Since Linux 3.11 their meaning changes
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always 12 at most architectures). Since Linux 3.11 their meaning changes
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after first clear of soft-dirty bits. Since Linux 4.2 they are used for
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after first clear of soft-dirty bits. Since Linux 4.2 they are used for
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flags unconditionally.
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flags unconditionally.
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Pagemap Scan IOCTL
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==================
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The ``PAGEMAP_SCAN`` IOCTL on the pagemap file can be used to get or optionally
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clear the info about page table entries. The following operations are supported
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in this IOCTL:
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- Scan the address range and get the memory ranges matching the provided criteria.
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This is performed when the output buffer is specified.
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- Write-protect the pages. The ``PM_SCAN_WP_MATCHING`` is used to write-protect
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the pages of interest. The ``PM_SCAN_CHECK_WPASYNC`` aborts the operation if
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non-Async Write Protected pages are found. The ``PM_SCAN_WP_MATCHING`` can be
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used with or without ``PM_SCAN_CHECK_WPASYNC``.
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- Both of those operations can be combined into one atomic operation where we can
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get and write protect the pages as well.
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Following flags about pages are currently supported:
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- ``PAGE_IS_WPALLOWED`` - Page has async-write-protection enabled
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- ``PAGE_IS_WRITTEN`` - Page has been written to from the time it was write protected
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- ``PAGE_IS_FILE`` - Page is file backed
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- ``PAGE_IS_PRESENT`` - Page is present in the memory
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- ``PAGE_IS_SWAPPED`` - Page is in swapped
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- ``PAGE_IS_PFNZERO`` - Page has zero PFN
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- ``PAGE_IS_HUGE`` - Page is THP or Hugetlb backed
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The ``struct pm_scan_arg`` is used as the argument of the IOCTL.
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1. The size of the ``struct pm_scan_arg`` must be specified in the ``size``
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field. This field will be helpful in recognizing the structure if extensions
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are done later.
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2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_WP_MATCHING``
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and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get
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operation is optionally performed depending upon if the output buffer is
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provided or not.
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3. The range is specified through ``start`` and ``end``.
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4. The walk can abort before visiting the complete range such as the user buffer
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can get full etc. The walk ending address is specified in``end_walk``.
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5. The output buffer of ``struct page_region`` array and size is specified in
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``vec`` and ``vec_len``.
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6. The optional maximum requested pages are specified in the ``max_pages``.
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7. The masks are specified in ``category_mask``, ``category_anyof_mask``,
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``category_inverted`` and ``return_mask``.
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Find pages which have been written and WP them as well::
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struct pm_scan_arg arg = {
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.size = sizeof(arg),
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.flags = PM_SCAN_CHECK_WPASYNC | PM_SCAN_CHECK_WPASYNC,
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..
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.category_mask = PAGE_IS_WRITTEN,
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.return_mask = PAGE_IS_WRITTEN,
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};
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Find pages which have been written, are file backed, not swapped and either
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present or huge::
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struct pm_scan_arg arg = {
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.size = sizeof(arg),
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.flags = 0,
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..
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.category_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED,
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.category_inverted = PAGE_IS_SWAPPED,
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.category_anyof_mask = PAGE_IS_PRESENT | PAGE_IS_HUGE,
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.return_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED |
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PAGE_IS_PRESENT | PAGE_IS_HUGE,
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};
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The ``PAGE_IS_WRITTEN`` flag can be considered as a better-performing alternative
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of soft-dirty flag. It doesn't get affected by VMA merging of the kernel and hence
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the user can find the true soft-dirty pages in case of normal pages. (There may
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still be extra dirty pages reported for THP or Hugetlb pages.)
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"PAGE_IS_WRITTEN" category is used with uffd write protect-enabled ranges to
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implement memory dirty tracking in userspace:
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1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall.
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2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features
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are set by ``UFFDIO_API`` IOCTL.
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3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
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through ``UFFDIO_REGISTER`` IOCTL.
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4. Then any part of the registered memory or the whole memory region must
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be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_WP_MATCHING``
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or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the
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same operation. The former is better in terms of performance.
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5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which
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have been written to since they were last marked and/or optionally write protect
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the pages as well.
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