[PATCH v6 3/3] mm/page_owner: document page_owner filter features

[Zhen Ni <zhen.ni@easystack.cn>]

Add documentation for the page_owner filter functionality, including:
- Print mode filter (full stack vs stack handle)
- NUMA node filter (single node, multiple nodes, ranges)
- Usage examples for both filters

Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
---

Changes in v6:
- No code changes

Changes in v5:
- No code changes

Changes in v4:
- Update print_mode documentation to reflect string-based interface
  * Change from "0/1" to "full_stack"/"stack_handle"
  * Add bracket notation example: "[full_stack] stack_handle"
- Update NUMA filter documentation
  * Remove "-1" example
  * Add empty string as clear method
- Fix indentation: use tabs instead of spaces in code examples

Changes in v3:
- New patch to document filter features as requested by Andrew Morton

v5: https://lore.kernel.org/linux-mm/20260507064643.179187-4-zhen.ni@easystack.cn/
v4: https://lore.kernel.org/linux-mm/20260430163247.13628-4-zhen.ni@easystack.cn/
v3: https://lore.kernel.org/linux-mm/20260428071112.1420380-5-zhen.ni@easystack.cn/
---
 Documentation/mm/page_owner.rst | 61 ++++++++++++++++++++++++++++++++-
 1 file changed, 60 insertions(+), 1 deletion(-)

diff --git a/Documentation/mm/page_owner.rst b/Documentation/mm/page_owner.rst
index 6b12f3b007ec..178bacfbb3fd 100644
--- a/Documentation/mm/page_owner.rst
+++ b/Documentation/mm/page_owner.rst
@@ -74,7 +74,17 @@ Usage
 
 3) Do the job that you want to debug.
 
-4) Analyze information from page owner::
+4) (Optional) Use filters to focus on specific memory allocations::
+
+	cd /sys/kernel/debug/page_owner_filter
+
+	# Print only stack handles instead of full traces
+	echo stack_handle > print_mode
+
+	# Filter by NUMA nodes
+	echo "0,2-3" > nid
+
+5) Analyze information from page owner::
 
 	cat /sys/kernel/debug/page_owner_stacks/show_stacks > stacks.txt
 	cat stacks.txt
@@ -238,6 +248,55 @@ Usage
 				./page_owner_sort <input> <output> --tgid=1,2,3
 				./page_owner_sort <input> <output> --name name1,name2
 
+Page Owner Filters
+==================
+
+The page_owner feature provides filtering capabilities to focus on specific
+memory allocations (e.g., by NUMA node). Filters are controlled through debugfs
+files in ``/sys/kernel/debug/page_owner_filter/``.
+
+Print Mode Filter
+-----------------
+
+The ``print_mode`` file controls the level of detail in stack trace output.
+
+Available modes:
+
+- ``full_stack`` (default): Print full stack traces
+- ``stack_handle``: Print only stack handles
+
+Reading the file shows the current mode with brackets around the active option::
+
+	cat /sys/kernel/debug/page_owner_filter/print_mode
+	[full_stack] stack_handle
+
+The ``stack_handle`` mode significantly reduces output size. Instead of full
+stack traces, it prints only the handle number::
+
+	Page allocated via order 0, mask 0x42800(GFP_NOWAIT|__GFP_COMP),
+	pid 1, tgid 1 (systemd), ts 349667370 ns
+	PFN 0xa00a2 type Unmovable Block 1280 type Unmovable
+	Flags 0x33fffe0000004124(...)
+	handle: 17432583
+
+To retrieve the full stack trace for a handle, use::
+
+	cat /sys/kernel/debug/page_owner_stacks/show_stacks_handles
+
+NUMA Node Filter
+----------------
+
+The ``nid`` file filters pages by NUMA node. This is useful for NUMA-aware
+environments to analyze node-specific memory allocation.
+
+Supported input formats:
+
+- Single node: ``echo "2" > nid``
+- Multiple nodes: ``echo "0,2,3" > nid``
+- Node range: ``echo "0-3" > nid``
+- Mixed format: ``echo "0,2-4,7" > nid``
+- Clear filter: ``echo > nid`` (empty string)
+
 STANDARD FORMAT SPECIFIERS
 ==========================
 ::
-- 
2.20.1