tools/mm/page_owner_sort: fix --sort option being silently ignored

Patch series "tools/mm/page_owner_sort: fix --sort, add module filter,
improve usage", v3.

This series improves the page_owner_sort tool with a bug fix, a new
module-name feature, and better usage text.

Patch 1 fixes a long-standing bug where --sort was silently ignored when
used without a short option (-a, -m, -p, etc.).  The COMP_NO_FLAG case
fell through to COMP_NUM and overwrote the sort conditions configured by
parse_sort_args().

Patch 2 adds kernel module name support for sort, cull, and filter
operations.  Page owner stack traces already contain module names in the
"function+0xNN/0xNN [module]" format produced by %pS, but page_owner_sort
had no way to use them.  Records without module frames are assigned
"vmlinux".

  # Aggregate page usage per module
  ./page_owner_sort input.txt output.txt --cull=mod

  # Filter to records from xfs module only
  ./page_owner_sort input.txt output.txt --module xfs

  # Sort by module name, then by pid descending
  ./page_owner_sort input.txt output.txt --sort=mod,-pid

Patch 3 lists all available sort keys with abbreviations and examples
directly in the --sort help section so users no longer need to read the
source to discover valid keys.


This patch (of 3):

When --sort is used without any short option (-a, -m, -p, etc.),
compare_flag remains COMP_NO_FLAG.  The switch (compare_flag) then falls
through to the COMP_NUM case and calls set_single_cmp(), which
unconditionally overwrites the sort conditions that parse_sort_args()
already configured.  This makes --sort silently ineffective unless a short
option is also supplied.

Split COMP_NO_FLAG out of the COMP_NUM fallthrough so that --sort is
respected when no short option is present.

Reproduction:
  # Before fix: ascending order (ignored --sort=-pid)
  ./page_owner_sort --sort=-pid input.txt output.txt
  # After fix: descending order as expected

Link: https://lore.kernel.org/20260819021611.2910835-1-ye.liu@linux.dev
Link: https://lore.kernel.org/20260819021611.2910835-2-ye.liu@linux.dev
Signed-off-by: Ye Liu <liuye@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Liu Jing <liujing@cmss.chinamobile.com>
Cc: Yichong Chen <chenyichong@uniontech.com>

tools/mm/page_owner_sort.c

@@ -821,6 +821,9 @@ int main(int argc, char **argv)
 		set_single_cmp(compare_stacktrace, SORT_ASC);
 		break;
 	case COMP_NO_FLAG:
+		if (sc.size > 0)
+			break;
+		/* fallthrough */
 	case COMP_NUM:
 		set_single_cmp(compare_num, SORT_DESC);
 		break;