mm/shmem: add mTHP swpout fallback statistics in shmem_writeout()

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().

Link: https://lkml.kernel.org/r/20251215024632.250149-1-tongweilin@linux.alibaba.com
Signed-off-by: Weilin Tong <tongweilin@linux.alibaba.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index ec6c01378e9d2bd47db9d7506e4d6a565e092185..15c2943140cae7009d2fcc9f25db00333471b985 100644 (file)
--- 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);
        }