[PATCH v3 0/4] mm/page_owner: add filter infrastructure for print_mode and NUMA filtering

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

This patch series introduces filtering capabilities to the page_owner
feature to address storage and performance challenges in production
environments.

Changes from v2:
- Remove READ_ONCE/WRITE_ONCE for nodemask_t (fixes compilation errors)
  * nodemask_t is a large structure (128 bytes) that triggers compile-time asserts
  * Direct assignment is safe for this use case
- Add comment explaining input length calculation formula
  * 6 bytes = ",NNNNN" (comma + 5-digit node number)
- Simplify "-1" check using kstrtoint() instead of dual strcmp()
- Move nodemask_t mask read outside PFN iteration loop for performance
  * Avoids 128-byte structure copy on each iteration
- Add documentation for filter features (patch 4/4)

Changes from v1:
- Renamed 'compact' to 'print_mode' with enum type for better clarity
  * PAGE_OWNER_PRINT_FULL_STACK (0): print full stack traces
  * PAGE_OWNER_PRINT_STACK_HANDLE (1): print only stack handles
- Changed NUMA filter from single node to nodelist with bitmask support
  * Uses nodelist_parse() to support "0", "0,2", "0-3", "0,2-4,7" formats
  * Uses nodemask_t internally for efficient multi-node filtering
  * Output uses %*pbl format (e.g., "0-2", "0,2-4,7")
- Improved memory handling in nid_filter_write using dynamic allocation
  * Limit: (100 + 6 * MAX_NUMNODES) to handle worst-case input

These changes address feedback from v2 review:
- AI review tool (sashiko.dev) identified READ_ONCE/WRITE_ONCE issue with nodemask_t
- Andrew Morton requested documentation for filter features
- Input length calculation justification
- Code simplification using kstrtoint()
- Performance optimization for mask read

Problem Statement
=================

In production environments with large memory configurations (e.g., 250GB+),
collecting page_owner information often results in files ranging from
several gigabytes to over 10GB. This creates significant challenges:

1. Storage pressure on production systems
2. Difficulty transferring large files from production environments
3. Post-processing overhead with tools/mm/page_owner_sort.c

The primary contributor to file size is redundant stack trace
information. While the kernel already deduplicates stacks via
stackdepot, page_owner retrieves and stores full stack traces for
each page, only to deduplicate them again during post-processing.

Additionally, in NUMA-aware environments (e.g., DPDK-based cloud
deployments where QEMU processes are bound to specific NUMA nodes),
OOM events are often node-specific rather than system-wide.
Currently, page_owner cannot filter by NUMA node, forcing users to
collect and analyze data for all nodes.

Solution
========

This patch series introduces a flexible filter infrastructure with
two initial filters:

1. **Print Mode Filter**: Outputs only stack handles instead of
   full stack traces. The handle-to-stack mapping can be retrieved
   from the existing show_stacks_handles interface. This dramatically
   reduces output size while preserving all allocation metadata.

2. **NUMA Node Filter**: Allows filtering pages by specific NUMA node(s)
   using flexible nodelist format, enabling targeted analysis of memory
   issues in NUMA-aware deployments.

Implementation
==============

The series is structured as follows:

- Patch 1: Add filter infrastructure (data structures and
  debugfs directory)
- Patch 2: Implement print_mode filter
- Patch 3: Implement NUMA node filter with nodelist support
- Patch 4: Document filter features

Usage Example
=============

Enable print_mode and filter for NUMA nodes 0,2-3:

    # cd /sys/kernel/debug/page_owner_filter/
    # echo 1 > print_mode
    # echo "0,2-3" > nid
    # cat /sys/kernel/debug/page_owner > page_owner.txt

Sample print_mode output (showing handles only):

    Page allocated via order 0, mask 0x0(), pid 0, tgid 0 (swapper),
    ts 0 ns PFN 0x40000 type Unmovable Block 512 type Unmovable
    Flags 0x3fffe0000000000(node=0|zone=0|lastcpupid=0x1ffff)
    handle: 1048577

    Page allocated via order 0, mask 0x252000(__GFP_NOWARN|
    __GFP_NORETRY|__GFP_COMP|__GFP_THISNODE), pid 0, tgid 0 (swapper),
    ts 0 ns PFN 0x40002 type Unmovable Block 512 type Unmovable
    Flags 0x23fffe0000000200(workingset|node=0|zone=0|lastcpupid=0x1ffff)
    handle: 1048577

Testing
=======

Tested on a system with multiple NUMA nodes. Verified that:
- Filters work independently and in combination
- Print_mode output correlates correctly with show_stacks_handles
- Default behavior (filters disabled) remains unchanged
- NUMA filter works with single node, multiple nodes, and ranges
- Code compiles without warnings or errors (allmodconfig tested)

Example test session:
    # cat print_mode
    0
    # echo "0,1-2" > nid
    # cat nid
    0-2
    # echo "0,2-3" > nid
    # cat nid
    0,2-3
    # echo 1 > print_mode
    # head -n 100 /sys/kernel/debug/page_owner
    [Shows compact mode output with handles only]

Future Enhancements
===================

The filter infrastructure is designed to be extensible. Potential
future filters could include:
- PID/TGID filtering
- Time range filtering (allocation timestamp windows)
- GFP flag filtering
- Migration type filtering

Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
---

Zhen Ni (4):
  mm/page_owner: add filter infrastructure
  mm/page_owner: add print_mode filter
  mm/page_owner: add NUMA node filter with nodelist support
  mm/page_owner: document page_owner filter features

 Documentation/mm/page_owner.rst |  55 +++++++++++++-
 mm/page_owner.c                 | 130 +++++++++++++++++++++++++++++++-
 2 files changed, 182 insertions(+), 3 deletions(-)

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2.20.1