Merge tag 'mm-stable-2023-11-01-14-33' of git://git.kernel.org/pub/sc…

…m/linux/kernel/git/akpm/mm Pull MM updates from Andrew Morton: "Many singleton patches against the MM code. The patch series which are included in this merge do the following: - Kemeng Shi has contributed some compation maintenance work in the series 'Fixes and cleanups to compaction' - Joel Fernandes has a patchset ('Optimize mremap during mutual alignment within PMD') which fixes an obscure issue with mremap()'s pagetable handling during a subsequent exec(), based upon an implementation which Linus suggested - More DAMON/DAMOS maintenance and feature work from SeongJae Park i the following patch series: mm/damon: misc fixups for documents, comments and its tracepoint mm/damon: add a tracepoint for damos apply target regions mm/damon: provide pseudo-moving sum based access rate mm/damon: implement DAMOS apply intervals mm/damon/core-test: Fix memory leaks in core-test mm/damon/sysfs-schemes: Do DAMOS tried regions update for only one apply interval - In the series 'Do not try to access unaccepted memory' Adrian Hunter provides some fixups for the recently-added 'unaccepted memory' feature. To increase the feature's checking coverage. 'Plug a few gaps where RAM is exposed without checking if it is unaccepted memory' - In the series 'cleanups for lockless slab shrink' Qi Zheng has done some maintenance work which is preparation for the lockless slab shrinking code - Qi Zheng has redone the earlier (and reverted) attempt to make slab shrinking lockless in the series 'use refcount+RCU method to implement lockless slab shrink' - David Hildenbrand contributes some maintenance work for the rmap code in the series 'Anon rmap cleanups' - Kefeng Wang does more folio conversions and some maintenance work in the migration code. Series 'mm: migrate: more folio conversion and unification' - Matthew Wilcox has fixed an issue in the buffer_head code which was causing long stalls under some heavy memory/IO loads. Some cleanups were added on the way. Series 'Add and use bdev_getblk()' - In the series 'Use nth_page() in place of direct struct page manipulation' Zi Yan has fixed a potential issue with the direct manipulation of hugetlb page frames - In the series 'mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO' has improved our handling of gigantic pages in the hugetlb vmmemmep optimizaton code. This provides significant boot time improvements when significant amounts of gigantic pages are in use - Matthew Wilcox has sent the series 'Small hugetlb cleanups' - code rationalization and folio conversions in the hugetlb code - Yin Fengwei has improved mlock()'s handling of large folios in the series 'support large folio for mlock' - In the series 'Expose swapcache stat for memcg v1' Liu Shixin has added statistics for memcg v1 users which are available (and useful) under memcg v2 - Florent Revest has enhanced the MDWE (Memory-Deny-Write-Executable) prctl so that userspace may direct the kernel to not automatically propagate the denial to child processes. The series is named 'MDWE without inheritance' - Kefeng Wang has provided the series 'mm: convert numa balancing functions to use a folio' which does what it says - In the series 'mm/ksm: add fork-exec support for prctl' Stefan Roesch makes is possible for a process to propagate KSM treatment across exec() - Huang Ying has enhanced memory tiering's calculation of memory distances. This is used to permit the dax/kmem driver to use 'high bandwidth memory' in addition to Optane Data Center Persistent Memory Modules (DCPMM). The series is named 'memory tiering: calculate abstract distance based on ACPI HMAT' - In the series 'Smart scanning mode for KSM' Stefan Roesch has optimized KSM by teaching it to retain and use some historical information from previous scans - Yosry Ahmed has fixed some inconsistencies in memcg statistics in the series 'mm: memcg: fix tracking of pending stats updates values' - In the series 'Implement IOCTL to get and optionally clear info about PTEs' Peter Xu has added an ioctl to /proc/<pid>/pagemap which permits us to atomically read-then-clear page softdirty state. This is mainly used by CRIU - Hugh Dickins contributed the series 'shmem,tmpfs: general maintenance', a bunch of relatively minor maintenance tweaks to this code - Matthew Wilcox has increased the use of the VMA lock over file-backed page faults in the series 'Handle more faults under the VMA lock'. Some rationalizations of the fault path became possible as a result - In the series 'mm/rmap: convert page_move_anon_rmap() to folio_move_anon_rmap()' David Hildenbrand has implemented some cleanups and folio conversions - In the series 'various improvements to the GUP interface' Lorenzo Stoakes has simplified and improved the GUP interface with an eye to providing groundwork for future improvements - Andrey Konovalov has sent along the series 'kasan: assorted fixes and improvements' which does those things - Some page allocator maintenance work from Kemeng Shi in the series 'Two minor cleanups to break_down_buddy_pages' - In thes series 'New selftest for mm' Breno Leitao has developed another MM self test which tickles a race we had between madvise() and page faults - In the series 'Add folio_end_read' Matthew Wilcox provides cleanups and an optimization to the core pagecache code - Nhat Pham has added memcg accounting for hugetlb memory in the series 'hugetlb memcg accounting' - Cleanups and rationalizations to the pagemap code from Lorenzo Stoakes, in the series 'Abstract vma_merge() and split_vma()' - Audra Mitchell has fixed issues in the procfs page_owner code's new timestamping feature which was causing some misbehaviours. In the series 'Fix page_owner's use of free timestamps' - Lorenzo Stoakes has fixed the handling of new mappings of sealed files in the series 'permit write-sealed memfd read-only shared mappings' - Mike Kravetz has optimized the hugetlb vmemmap optimization in the series 'Batch hugetlb vmemmap modification operations' - Some buffer_head folio conversions and cleanups from Matthew Wilcox in the series 'Finish the create_empty_buffers() transition' - As a page allocator performance optimization Huang Ying has added automatic tuning to the allocator's per-cpu-pages feature, in the series 'mm: PCP high auto-tuning' - Roman Gushchin has contributed the patchset 'mm: improve performance of accounted kernel memory allocations' which improves their performance by ~30% as measured by a micro-benchmark - folio conversions from Kefeng Wang in the series 'mm: convert page cpupid functions to folios' - Some kmemleak fixups in Liu Shixin's series 'Some bugfix about kmemleak' - Qi Zheng has improved our handling of memoryless nodes by keeping them off the allocation fallback list. This is done in the series 'handle memoryless nodes more appropriately' - khugepaged conversions from Vishal Moola in the series 'Some khugepaged folio conversions'" [ bcachefs conflicts with the dynamically allocated shrinkers have been resolved as per Stephen Rothwell in https://lore.kernel.org/all/[email protected]/ with help from Qi Zheng. The clone3 test filtering conflict was half-arsed by yours truly ] * tag 'mm-stable-2023-11-01-14-33' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (406 commits) mm/damon/sysfs: update monitoring target regions for online input commit mm/damon/sysfs: remove requested targets when online-commit inputs selftests: add a sanity check for zswap Documentation: maple_tree: fix word spelling error mm/vmalloc: fix the unchecked dereference warning in vread_iter() zswap: export compression failure stats Documentation: ubsan: drop "the" from article title mempolicy: migration attempt to match interleave nodes mempolicy: mmap_lock is not needed while migrating folios mempolicy: alloc_pages_mpol() for NUMA policy without vma mm: add page_rmappable_folio() wrapper mempolicy: remove confusing MPOL_MF_LAZY dead code mempolicy: mpol_shared_policy_init() without pseudo-vma mempolicy trivia: use pgoff_t in shared mempolicy tree mempolicy trivia: slightly more consistent naming mempolicy trivia: delete those ancient pr_debug()s mempolicy: fix migrate_pages(2) syscall return nr_failed kernfs: drop shared NUMA mempolicy hooks hugetlbfs: drop shared NUMA mempolicy pretence mm/damon/sysfs-test: add a unit test for damon_sysfs_set_targets() ...
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diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-damon b/Documentation/ABI/testing/sysfs-kernel-mm-damon
@@ -151,6 +151,13 @@ Contact:	SeongJae Park <[email protected]>
 Description:	Writing to and reading from this file sets and gets the action
 		of the scheme.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/apply_interval_us
+Date:		Sep 2023
+Contact:	SeongJae Park <[email protected]>
+Description:	Writing a value to this file sets the action apply interval of
+		the scheme in microseconds.  Reading this file returns the
+		value.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/access_pattern/sz/min
 Date:		Mar 2022
 Contact:	SeongJae Park <[email protected]>

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -551,6 +551,7 @@ memory.stat file includes following statistics:
                     event happens each time a page is unaccounted from the
                     cgroup.
     swap            # of bytes of swap usage
+    swapcached      # of bytes of swap cached in memory
     dirty           # of bytes that are waiting to get written back to the disk.
     writeback       # of bytes of file/anon cache that are queued for syncing to
                     disk.

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
@@ -210,6 +210,35 @@ cgroup v2 currently supports the following mount options.
         relying on the original semantics (e.g. specifying bogusly
         high 'bypass' protection values at higher tree levels).
 
+  memory_hugetlb_accounting
+        Count HugeTLB memory usage towards the cgroup's overall
+        memory usage for the memory controller (for the purpose of
+        statistics reporting and memory protetion). This is a new
+        behavior that could regress existing setups, so it must be
+        explicitly opted in with this mount option.
+
+        A few caveats to keep in mind:
+
+        * There is no HugeTLB pool management involved in the memory
+          controller. The pre-allocated pool does not belong to anyone.
+          Specifically, when a new HugeTLB folio is allocated to
+          the pool, it is not accounted for from the perspective of the
+          memory controller. It is only charged to a cgroup when it is
+          actually used (for e.g at page fault time). Host memory
+          overcommit management has to consider this when configuring
+          hard limits. In general, HugeTLB pool management should be
+          done via other mechanisms (such as the HugeTLB controller).
+        * Failure to charge a HugeTLB folio to the memory controller
+          results in SIGBUS. This could happen even if the HugeTLB pool
+          still has pages available (but the cgroup limit is hit and
+          reclaim attempt fails).
+        * Charging HugeTLB memory towards the memory controller affects
+          memory protection and reclaim dynamics. Any userspace tuning
+          (of low, min limits for e.g) needs to take this into account.
+        * HugeTLB pages utilized while this option is not selected
+          will not be tracked by the memory controller (even if cgroup
+          v2 is remounted later on).
+
 
 Organizing Processes and Threads
 --------------------------------
@@ -1539,6 +1568,15 @@ PAGE_SIZE multiple when read back.
 		collapsing an existing range of pages. This counter is not
 		present when CONFIG_TRANSPARENT_HUGEPAGE is not set.
 
+	  thp_swpout (npn)
+		Number of transparent hugepages which are swapout in one piece
+		without splitting.
+
+	  thp_swpout_fallback (npn)
+		Number of transparent hugepages which were split before swapout.
+		Usually because failed to allocate some continuous swap space
+		for the huge page.
+
   memory.numa_stat
 	A read-only nested-keyed file which exists on non-root cgroups.
 

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
@@ -20,18 +20,18 @@ DAMON provides below interfaces for different users.
   you can write and use your personalized DAMON sysfs wrapper programs that
   reads/writes the sysfs files instead of you.  The `DAMON user space tool
   <https://github.com/awslabs/damo>`_ is one example of such programs.
-- *debugfs interface. (DEPRECATED!)*
-  :ref:`This <debugfs_interface>` is almost identical to :ref:`sysfs interface
-  <sysfs_interface>`.  This is deprecated, so users should move to the
-  :ref:`sysfs interface <sysfs_interface>`.  If you depend on this and cannot
-  move, please report your usecase to [email protected] and
-  [email protected].
 - *Kernel Space Programming Interface.*
   :doc:`This </mm/damon/api>` is for kernel space programmers.  Using this,
   users can utilize every feature of DAMON most flexibly and efficiently by
   writing kernel space DAMON application programs for you.  You can even extend
   DAMON for various address spaces.  For detail, please refer to the interface
   :doc:`document </mm/damon/api>`.
+- *debugfs interface. (DEPRECATED!)*
+  :ref:`This <debugfs_interface>` is almost identical to :ref:`sysfs interface
+  <sysfs_interface>`.  This is deprecated, so users should move to the
+  :ref:`sysfs interface <sysfs_interface>`.  If you depend on this and cannot
+  move, please report your usecase to [email protected] and
+  [email protected].
 
 .. _sysfs_interface:
 
@@ -76,7 +76,7 @@ comma (","). ::
     │ │ │ │ │ │ │ │ ...
     │ │ │ │ │ │ ...
     │ │ │ │ │ schemes/nr_schemes
-    │ │ │ │ │ │ 0/action
+    │ │ │ │ │ │ 0/action,apply_interval_us
     │ │ │ │ │ │ │ access_pattern/
     │ │ │ │ │ │ │ │ sz/min,max
     │ │ │ │ │ │ │ │ nr_accesses/min,max
@@ -105,14 +105,12 @@ having the root permission could use this directory.
 kdamonds/
 ---------
 
-The monitoring-related information including request specifications and results
-are called DAMON context.  DAMON executes each context with a kernel thread
-called kdamond, and multiple kdamonds could run in parallel.
-
 Under the ``admin`` directory, one directory, ``kdamonds``, which has files for
-controlling the kdamonds exist.  In the beginning, this directory has only one
-file, ``nr_kdamonds``.  Writing a number (``N``) to the file creates the number
-of child directories named ``0`` to ``N-1``.  Each directory represents each
+controlling the kdamonds (refer to
+:ref:`design <damon_design_execution_model_and_data_structures>` for more
+details) exists.  In the beginning, this directory has only one file,
+``nr_kdamonds``.  Writing a number (``N``) to the file creates the number of
+child directories named ``0`` to ``N-1``.  Each directory represents each
 kdamond.
 
 kdamonds/<N>/
@@ -150,9 +148,10 @@ kdamonds/<N>/contexts/
 
 In the beginning, this directory has only one file, ``nr_contexts``.  Writing a
 number (``N``) to the file creates the number of child directories named as
-``0`` to ``N-1``.  Each directory represents each monitoring context.  At the
-moment, only one context per kdamond is supported, so only ``0`` or ``1`` can
-be written to the file.
+``0`` to ``N-1``.  Each directory represents each monitoring context (refer to
+:ref:`design <damon_design_execution_model_and_data_structures>` for more
+details).  At the moment, only one context per kdamond is supported, so only
+``0`` or ``1`` can be written to the file.
 
 .. _sysfs_contexts:
 
@@ -270,8 +269,8 @@ schemes/<N>/
 ------------
 
 In each scheme directory, five directories (``access_pattern``, ``quotas``,
-``watermarks``, ``filters``, ``stats``, and ``tried_regions``) and one file
-(``action``) exist.
+``watermarks``, ``filters``, ``stats``, and ``tried_regions``) and two files
+(``action`` and ``apply_interval``) exist.
 
 The ``action`` file is for setting and getting the scheme's :ref:`action
 <damon_design_damos_action>`.  The keywords that can be written to and read
@@ -297,6 +296,9 @@ Note that support of each action depends on the running DAMON operations set
  - ``stat``: Do nothing but count the statistics.
    Supported by all operations sets.
 
+The ``apply_interval_us`` file is for setting and getting the scheme's
+:ref:`apply_interval <damon_design_damos>` in microseconds.
+
 schemes/<N>/access_pattern/
 ---------------------------
 
@@ -392,7 +394,7 @@ pages of all memory cgroups except ``/having_care_already``.::
     echo N > 1/matching
 
 Note that ``anon`` and ``memcg`` filters are currently supported only when
-``paddr`` `implementation <sysfs_contexts>` is being used.
+``paddr`` :ref:`implementation <sysfs_contexts>` is being used.
 
 Also, memory regions that are filtered out by ``addr`` or ``target`` filters
 are not counted as the scheme has tried to those, while regions that filtered
@@ -430,9 +432,9 @@ that reading it returns the total size of the scheme tried regions, and creates
 directories named integer starting from ``0`` under this directory.  Each
 directory contains files exposing detailed information about each of the memory
 region that the corresponding scheme's ``action`` has tried to be applied under
-this directory, during next :ref:`aggregation interval
-<sysfs_monitoring_attrs>`.  The information includes address range,
-``nr_accesses``, and ``age`` of the region.
+this directory, during next :ref:`apply interval <damon_design_damos>` of the
+corresponding scheme.  The information includes address range, ``nr_accesses``,
+and ``age`` of the region.
 
 Writing ``update_schemes_tried_bytes`` to the relevant ``kdamonds/<N>/state``
 file will only update the ``total_bytes`` file, and will not create the
@@ -495,6 +497,62 @@ Please note that it's highly recommended to use user space tools like `damo
 <https://github.com/awslabs/damo>`_ rather than manually reading and writing
 the files as above.  Above is only for an example.
 
+.. _tracepoint:
+
+Tracepoints for Monitoring Results
+==================================
+
+Users can get the monitoring results via the :ref:`tried_regions
+<sysfs_schemes_tried_regions>`.  The interface is useful for getting a
+snapshot, but it could be inefficient for fully recording all the monitoring
+results.  For the purpose, two trace points, namely ``damon:damon_aggregated``
+and ``damon:damos_before_apply``, are provided.  ``damon:damon_aggregated``
+provides the whole monitoring results, while ``damon:damos_before_apply``
+provides the monitoring results for regions that each DAMON-based Operation
+Scheme (:ref:`DAMOS <damon_design_damos>`) is gonna be applied.  Hence,
+``damon:damos_before_apply`` is more useful for recording internal behavior of
+DAMOS, or DAMOS target access
+:ref:`pattern <damon_design_damos_access_pattern>` based query-like efficient
+monitoring results recording.
+
+While the monitoring is turned on, you could record the tracepoint events and
+show results using tracepoint supporting tools like ``perf``.  For example::
+
+    # echo on > monitor_on
+    # perf record -e damon:damon_aggregated &
+    # sleep 5
+    # kill 9 $(pidof perf)
+    # echo off > monitor_on
+    # perf script
+    kdamond.0 46568 [027] 79357.842179: damon:damon_aggregated: target_id=0 nr_regions=11 122509119488-135708762112: 0 864
+    [...]
+
+Each line of the perf script output represents each monitoring region.  The
+first five fields are as usual other tracepoint outputs.  The sixth field
+(``target_id=X``) shows the ide of the monitoring target of the region.  The
+seventh field (``nr_regions=X``) shows the total number of monitoring regions
+for the target.  The eighth field (``X-Y:``) shows the start (``X``) and end
+(``Y``) addresses of the region in bytes.  The ninth field (``X``) shows the
+``nr_accesses`` of the region (refer to
+:ref:`design <damon_design_region_based_sampling>` for more details of the
+counter).  Finally the tenth field (``X``) shows the ``age`` of the region
+(refer to :ref:`design <damon_design_age_tracking>` for more details of the
+counter).
+
+If the event was ``damon:damos_beofre_apply``, the ``perf script`` output would
+be somewhat like below::
+
+    kdamond.0 47293 [000] 80801.060214: damon:damos_before_apply: ctx_idx=0 scheme_idx=0 target_idx=0 nr_regions=11 121932607488-135128711168: 0 136
+    [...]
+
+Each line of the output represents each monitoring region that each DAMON-based
+Operation Scheme was about to be applied at the traced time.  The first five
+fields are as usual.  It shows the index of the DAMON context (``ctx_idx=X``)
+of the scheme in the list of the contexts of the context's kdamond, the index
+of the scheme (``scheme_idx=X``) in the list of the schemes of the context, in
+addition to the output of ``damon_aggregated`` tracepoint.
+
+
 .. _debugfs_interface:
 
 debugfs Interface (DEPRECATED!)
@@ -790,23 +848,3 @@ directory by putting the name of the context to the ``rm_contexts`` file. ::
 
 Note that ``mk_contexts``, ``rm_contexts``, and ``monitor_on`` files are in the
 root directory only.
-
-
-.. _tracepoint:
-
-Tracepoint for Monitoring Results
-=================================
-
-Users can get the monitoring results via the :ref:`tried_regions
-<sysfs_schemes_tried_regions>` or a tracepoint, ``damon:damon_aggregated``.
-While the tried regions directory is useful for getting a snapshot, the
-tracepoint is useful for getting a full record of the results.  While the
-monitoring is turned on, you could record the tracepoint events and show
-results using tracepoint supporting tools like ``perf``.  For example::
-
-    # echo on > monitor_on
-    # perf record -e damon:damon_aggregated &
-    # sleep 5
-    # kill 9 $(pidof perf)
-    # echo off > monitor_on
-    # perf script
diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
@@ -155,6 +155,15 @@ stable_node_chains_prune_millisecs
         scan. It's a noop if not a single KSM page hit the
         ``max_page_sharing`` yet.
 
+smart_scan
+        Historically KSM checked every candidate page for each scan. It did
+        not take into account historic information.  When smart scan is
+        enabled, pages that have previously not been de-duplicated get
+        skipped. How often these pages are skipped depends on how often
+        de-duplication has already been tried and failed. By default this
+        optimization is enabled.  The ``pages_skipped`` metric shows how
+        effective the setting is.
+
 The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
 
 general_profit
@@ -169,6 +178,8 @@ pages_unshared
         how many pages unique but repeatedly checked for merging
 pages_volatile
         how many pages changing too fast to be placed in a tree
+pages_skipped
+        how many pages did the "smart" page scanning algorithm skip
 full_scans
         how many times all mergeable areas have been scanned
 stable_node_chains

diff --git 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.