Hi Randy, Thank you for your review. On 6/14/23 9:20 AM, Randy Dunlap wrote: > Hi-- > > On 6/13/23 03:29, Muhammad Usama Anjum wrote: >> Add some explanation and method to use write-protection and written-to >> on memory range. >> >> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx> >> --- >> 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..3b977526d4b3 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``), > > written to I'll make updates on all the places you have suggested. > >> + 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 > > written to > >> + (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`` un-set, ``required_mask`` > > unset, > >> + 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 household chores (VMA > > housekeeping ? > >> +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 > > written to > >> +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 the any part of the registered memory or the whole memory region must > > "the" or "any". Probably "any". Not both. > >> + 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. > > written to > > -- BR, Muhammad Usama Anjum
