mm/page_owner: document page_owner filter

Add documentation for the page_owner_filter userspace tool and
kernel-level filtering features.

[rdunlap@infradead.org: avoid docs build warning]
  Link: https://lore.kernel.org/20260708213548.469155-1-rdunlap@infradead.org
Link: https://lore.kernel.org/20260707115411.1714314-5-zhen.ni@easystack.cn
Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/page_owner.rst

@@ -65,7 +65,15 @@ un-tracking state.
 Usage
 =====
 
-1) Build user-space helper::
+1) Build user-space helpers:
+::
+
+   To filter page_owner output:
+
+	cd tools/mm
+	make page_owner_filter
+
+   To sort and analyze page_owner output:
 
 	cd tools/mm
 	make page_owner_sort
@@ -74,7 +82,11 @@ Usage
 
 3) Do the job that you want to debug.
 
-4) Analyze information from page owner::
+4) (Optional) Filter page_owner output::
+
+	./page_owner_filter -m handle -n 0,1,2 > filtered_page_owner.txt
+
+5) Analyze information from page owner::
 
 	cat /sys/kernel/debug/page_owner_stacks/show_stacks > stacks.txt
 	cat stacks.txt
@@ -263,3 +275,65 @@ STANDARD FORMAT SPECIFIERS
 	f		free		whether the page has been released or not
 	st		stacktrace	stack trace of the page allocation
 	ator		allocator	memory allocator for pages
+
+Filtering page_owner output
+============================
+
+page_owner supports filtering output at the kernel level before reading,
+which reduces the amount of data that needs to be processed in userspace.
+
+The page_owner_filter tool provides a convenient interface for this filtering
+capability. It supports two types of filters:
+
+1. **print_mode filter**: Control what information is printed for each page
+	- ``stack``: Print full stack traces (default, compatible with existing usage)
+	- ``handle``: Print only stack handle numbers (much faster, smaller output)
+	- ``stack_handle``: Print both stack traces and handle numbers
+
+	The ``handle`` mode uses numeric identifiers instead of full stack traces.
+	The mapping from handles to actual stack traces can be obtained via the
+	show_stacks_handles interface.
+
+2. **NUMA node filter**: Filter pages by NUMA node ID
+	- Supports single node: ``-n 0``
+	- Multiple nodes: ``-n 0,1,2``
+	- Ranges: ``-n 0-3``
+	- Mixed format: ``-n 0,2-3,5``
+
+Usage examples::
+
+	# Filter by print mode
+	./page_owner_filter -m handle
+	./page_owner_filter -m stack_handle
+
+	# Filter by NUMA node
+	./page_owner_filter -n 0
+	./page_owner_filter -n 0-3
+
+	# Combined filters
+	./page_owner_filter -m stack -n 0,1,2
+	./page_owner_filter -m handle -n 0,2-3
+
+	# Save to file
+	./page_owner_filter -m handle -o filtered_output.txt
+
+The handle mode is particularly useful for monitoring and performance-critical
+scenarios as it dramatically reduces output size. Testing shows handle mode can
+reduce output size by ~66% (84MB vs 244MB) and improve read performance by ~4.4x
+compared to full stack output.
+
+The NUMA node filter is useful for NUMA-aware memory allocation analysis and debugging.
+
+Behind the scenes, page_owner_filter opens /sys/kernel/debug/page_owner and
+writes filter commands before reading the filtered output. The filtering uses
+per-file-descriptor state, allowing each open() to have independent filter settings.
+
+Each file descriptor maintains its own filter state, so you can have multiple
+independent filtering operations running concurrently. For example, in different
+terminals you can run different filters simultaneously::
+
+	# Terminal 1: Filter node 0
+	./page_owner_filter -n 0 > node0_output.txt
+
+	# Terminal 2: Filter node 1 (runs concurrently)
+	./page_owner_filter -n 1 > node1_output.txt