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@collabora.com
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.
- 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.
- 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.
- The range is specified through ``start`` and ``len``.
- The output buffer of ``struct page_region`` array and size is specified in
- ``vec`` and ``vec_len``.
- The optional maximum requested pages are specified in the ``max_pages``.
- The masks are specified in ``required_mask``, ``anyof_mask``,
- ``excluded_ mask`` and ``return_mask``.
- 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``.
- 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``.
- To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is
specified in ``required_mask`` and ``return_mask``. In addition tospecifying the output buffer in ``vec`` and length in ``vec_len``, the``PM_SCAN_OP_WP`` is specified in ``flags`` to perform write protecton 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:
- The userfaultfd file descriptor is created with ``userfaultfd`` syscall.
- The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features
- are set by ``UFFDIO_API`` IOCTL.
- The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
- through ``UFFDIO_REGISTER`` IOCTL.
- 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.
- 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