Add some explanation and method to use write-protection and written-to on memory range.
Signed-off-by: Muhammad Usama Anjum usama.anjum@collabora.com --- Changes in v30: - Minor changes
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..fe17cf2104265 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 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.