mm/page_owner: misc cleanups

This series collects a few cleanups for mm/page_owner.c that have been accumulated while reading through the file. There is no functional change -- the goal is to make the code easier to read and maintain.

Patch 1 consolidates three identical PageBuddy skip blocks into a single skip_buddy_pages() helper, eliminating the duplication and keeping the lockless-read comment in one place.

Patch 2 replaces the -1 magic number used for "never migrated" with a proper MR_NEVER member in enum migrate_reason, adds the corresponding "never_migrated" string in the MIGRATE_REASON trace macro, and updates the GDB page_owner script to use MR_NEVER so that lx-dump-page-owner correctly detects unmigrated pages.

Patch 3 follows up by converting the remaining 'int reason' parameters throughout the migration and hugetlb callchains to 'enum migrate_reason', making the type explicit and gaining compiler checking. The 'short last_migrate_reason' struct field in page_owner is intentionally left as 'short' since it is per-page metadata where size matters.

Patch 4 hoists the CONFIG_MEMCG guard out of print_page_owner_memcg()'s body so that the real implementation and the empty stub are two clearly separate definitions, the common kernel idiom.

Patch 5 adds a missing \n to the count_threshold debugfs attribute format string so that cat(1) output is properly terminated.

Patch 6 moves free_ts_nsec from the allocation summary line to the free section in __dump_page_owner(), grouping it with free_pid and free_tgid where it logically belongs. This also makes the dump output consistent with print_page_owner().

Patch 7 drops the redundant page_owner_ prefix from file-scoped static symbols (stack_fops, threshold_fops, etc.). Since they cannot collide across translation units, the prefix carries no information.

Patch 8 clamps the PFN advance in skip_buddy_pages() at the next MAX_ORDER_NR_PAGES boundary. The lockless buddy_order_unsafe() read can return a garbage order value if the page is concurrently allocated between the PageBuddy check and the private read, potentially causing the PFN to advance past the next bounadry whose pfn_valid() check would have caught an offline memory section. In read_page_owner(), which relies solely on boundary-aligned pfn_valid() to guard pfn_to_page(), this could lead to an unmapped mem_section access.

Patch 9 avoids two TOCTOU issues in print_page_owner_memcg() by reusing the page->memcg_data snapshot already taken via READ_ONCE at the top of the function throughout, instead of calling page_memcg_check() and PageMemcgKmem() which re-read page->memcg_data locklessly with VM_BUG_ON assertions. If the page is concurrently freed and reallocated as a THP tail or slab page between the initial guards and these later calls, those assertions can fire on CONFIG_DEBUG_VM=y builds. The OBJEXTS (slab) case is also simplified with an early return since objcg != memcg for slabs.

Ye Liu (9):
mm/page_owner: extract skip_buddy_pages() helper to unify buddy page
skipping
mm/page_owner: add MR_NEVER to enum migrate_reason and use it for
last_migrate_reason
mm: use enum migrate_reason instead of int for migration reason
parameters
mm/page_owner: hoist CONFIG_MEMCG to function level for
print_page_owner_memcg()
mm/page_owner: add missing newline to count_threshold format string mm/page_owner: move free_ts_nsec output to free section in

    __dump_page_owner()

mm/page_owner: drop redundant page_owner prefix from static symbols mm/page_owner: clamp skip_buddy_pages() PFN advance at
MAX_ORDER_NR_PAGES boundary
mm/page_owner: use memcg_data snapshot to avoid TOCTOU in
print_page_owner_memcg()

 include/linux/hugetlb.h         |   9 ++-
 include/linux/migrate.h         |   8 +-
 include/linux/migrate_mode.h    |   1 +
 include/linux/page_owner.h      |   7 +-
 include/trace/events/migrate.h  |  11 +--
 mm/hugetlb.c                    |   3 +-
 mm/migrate.c                    |  12 +--
 mm/page_owner.c                 | 135 ++++++++++++++++++--------------
 scripts/gdb/linux/page_owner.py |   4 +-
 9 files changed, 109 insertions(+), 81 deletions(-)

---
v6:
- Patch 3: drop unnecessary 'extern' from __folio_set_owner_migrate_reason() declaration in page_owner.h; Adjust the indentation; add Reviewed-by from Lorenzo Stoakes.
- Patch 8: clarify commit message wording; add Reviewed-by from Zi Yan and Vlastimil Babka.
- Patch 9: rename patch to cover both TOCTOU fixes; also replace page_memcg_check(page) with extracting objcg from the memcg_data snapshot to fix a second TOCTOU issue; add early return for the MEMCG_DATA_OBJEXTS (slab) case.
- Link: https://lore.kernel.org/all/[email protected]/
v5:
- Place the two patches corresponding to the Close connection, patch8 and patch9, together in this part.
- Close: https://lore.kernel.org/all/[email protected]/
- Link: https://lore.kernel.org/all/[email protected]/

v4:
- Patch 2: also update scripts/gdb/linux/page_owner.py to use MR_NEVER instead of the hardcoded -1.
- https://lore.kernel.org/all/[email protected]/

v3:
- Patch 2: add MR_NEVER directly to enum migrate_reason instead of using a local MIGRATE_REASON_NONE define (Zi Yan and Vlastimil Babka).
- Patch 3 (new): convert all 'int reason' parameters in the migration callchain to 'enum migrate_reason' so the type system reflects the actual semantics.
- Patch 7: make threshold_fops a single line.
- Link: https://lore.kernel.org/all/[email protected]/

v2:
- Add cover letter (no code changes).
- Link: https://lore.kernel.org/all/[email protected]/

2.43.0