[RFC PATCH] mm/shmem: add mTHP swpout fallback statistics in shmem_writeout()

[Weilin Tong <tongweilin@linux.alibaba.com>]

Currently, when shmem mTHPs are split and swapped out via shmem_writeout(),
there are no unified statistics to trace these mTHP swpout fallback events.
This makes it difficult to analyze the prevalence of mTHP splitting and
fallback during swap operations, which is important for memory diagnostics.

Here we add statistics counting for mTHP fallback to small pages
when splitting and swapping out in shmem_writeout().

Signed-off-by: Weilin Tong <tongweilin@linux.alibaba.com>
---
 mm/shmem.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 3f194c9842a8..aa624c447358 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1593,11 +1593,23 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 	}
 
 	if (split) {
+		int order;
+
 try_split:
+		order = folio_order(folio);
 		/* Ensure the subpages are still dirty */
 		folio_test_set_dirty(folio);
 		if (split_folio_to_list(folio, folio_list))
 			goto redirty;
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+		if (order >= HPAGE_PMD_ORDER) {
+			count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
+			count_vm_event(THP_SWPOUT_FALLBACK);
+		}
+#endif
+		count_mthp_stat(order, MTHP_STAT_SWPOUT_FALLBACK);
+
 		folio_clear_dirty(folio);
 	}
 
-- 
2.43.5