[PATCH v5 0/9] mm/page_owner: misc cleanups

[Ye Liu <ye.liu@linux.dev>]

v5:
 - Place the two patches corresponding to the Close connection, patch8 
   and patch9, together in this part.
 - Close: https://lore.kernel.org/all/20260625014708.87386-1-ye.liu@linux.dev/
 - Link: https://lore.kernel.org/all/20260701012239.315262-1-ye.liu@linux.dev/

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/20260630015331.147174-1-ye.liu@linux.dev/

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/20260626020522.28619-1-ye.liu@linux.dev/

v2:
 - Add cover letter (no code changes).
 - Link: https://lore.kernel.org/all/20260623065234.31866-2-ye.liu@linux.dev/

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 a TOCTOU VM_BUG_ON in print_page_owner_memcg() by
reusing the page->memcg_data snapshot already taken via READ_ONCE at
the top of the function, instead of calling PageMemcgKmem() which
re-reads folio->memcg_data and page->compound_head 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
this final call, those assertions can fire on CONFIG_DEBUG_VM=y builds.

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 instead of PageMemcgKmem() to
    avoid TOCTOU VM_BUG_ON

 include/linux/hugetlb.h         |   9 ++-                             
 include/linux/migrate.h         |   6 +-                              
 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                 | 127 ++++++++++++++++++--------------
 scripts/gdb/linux/page_owner.py |   4 +-                              
 9 files changed, 102 insertions(+), 78 deletions(-)                   


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2.43.0