[PATCH v25 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

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 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

rst
---
 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..4bdad8eff009 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``),
+  file mapped (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``), swapped
+  (``PAGE_IS_SWAPPED``) or page has pfn zero (``PAGE_IS_PFNZERO``).
+- Find pages which have been written to and/or write protect the pages atomically
+  (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 ``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 ``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`` unset, ``required_mask``
+        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 housekeeping chores
+(VMA 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
+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 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.
+ 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.
-- 
2.39.2