Re: [PATCH v28 5/6] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL

On Wed, 9 Aug 2023 at 08:16, Muhammad Usama Anjum usama.anjum@collabora.com wrote:

Add some explanation and method to use write-protection and written-to on memory range.

[...]

--- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -227,3 +227,67 @@ 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 Async Write-Protection enabled

• (``PAGE_IS_WPALLOWED``), 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``).

A recent addition -- PAGE_IS_HUGE -- is missing.

BTW, it could be easier to understand if the page categories were separated from the operation description and listed so that each has its own line and maybe a longer description where needed.

+- Find pages which have been written to and/or write protect

• (atomic ``PM_SCAN_WP_MATCHING + PM_SCAN_CHECK_WPASYNC``) the pages atomically.
• The (``PM_SCAN_WP_MATCHING``) is used to WP the matched pages. The
• (``PM_SCAN_CHECK_WPASYNC``) aborts the operation if non-Async-Write-Protected
• pages are found.

The operation the IOCTL does now is: "scan the process page tables and report memory ranges matching provided criteria '. Flags extend the operation: "PM_SCAN_WP_MATCHING write protects the memory reported" (it does it atomically, but this is just an optimization, isn't it? A process could gather the ranges, WP them, and then copy.) "PM_SCAN_CHECK_WPASYNC" aborts the scan early if a non-WP-able matching page is found.

+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 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 ``category_mask``, ``category_anyof_mask``,
• ``category_inverted`` 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, ``category_mask``
  

•    is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``, ``category_inverted`` 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, ``category_anyof_mask`` is set to  ``PAGE_IS_FILE | PAGE_IS_SWAPPED``.
  

• 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is

•   specified in ``category_mask`` and ``return_mask``. In addition to
  

•   specifying the output buffer in ``vec`` and length in ``vec_len``, the
  

•   ``PM_SCAN_WP_MATCHING`` is specified in ``flags`` to perform write protect
  

•   on the range as well.
  

Could this be rewritten as examples? E.g.:

Finding dirty file-backed pages:

struct pm_scan_arg arg = { .size = sizeof(arg), .flags = 0, ... .category_mask = ..., .return_mask = ... }; ssize_t n = ioctl(..., &arg);

Find dirty pages and write protect them in the same call:

arg = { ... }; do { ... ioctl(...) } while(...);

(The code snippets heavily commented.)

+The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct

"as a better-performing alternative"

+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 is still an optimization, e.g. in THP case there might be too many pages reported?

• 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.

This repeats the description of PM_SCAN_WP_MATCHING -- I suggest removing this part.

+To get information about which pages have been written to or optionally write +protect the pages, following must be performed first in order:

"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_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.

I guess that the UFFD performance could be fixed? But this part refers to the old PM_SCAN_OP_WP, so an updated example is needed.

• 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.

"find the pages written to since they were last write protected", but this sounds contradicting: we look for pages that were WP but written anyway. (IOW: marking write-protected is an implementation detail - the ioctl is to find pages that changed since they were last marked.) Maybe we should call the operation "marking CLEAN" or alike?

Best Regards Michał Mirosław