mm: vmscan: avoid split during shrink_folio_list()

Now that swap supports storing all mTHP sizes, avoid splitting large
folios before swap-out.  This benefits performance of the swap-out path by
eliding split_folio_to_list(), which is expensive, and also sets us up for
swapping in large folios in a future series.

If the folio is partially mapped, we continue to split it since we want to
avoid the extra IO overhead and storage of writing out pages
uneccessarily.

THP_SWPOUT and THP_SWPOUT_FALLBACK counters should continue to count
events only for PMD-mappable folios to avoid user confusion.  THP_SWPOUT
already has the appropriate guard.  Add a guard for THP_SWPOUT_FALLBACK. 
It may be appropriate to add per-size counters in future.

Link: https://lkml.kernel.org/r/20240408183946.2991168-7-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Barry Song <v-songbaohua@oppo.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Gao Xiang <xiang@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -1206,25 +1206,25 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 					if (!can_split_folio(folio, NULL))
 						goto activate_locked;
 					/*
-					 * Split folios without a PMD map right
-					 * away. Chances are some or all of the
-					 * tail pages can be freed without IO.
+					 * Split partially mapped folios right away.
+					 * We can free the unmapped pages without IO.
 					 */
-					if (!folio_entire_mapcount(folio) &&
-					    split_folio_to_list(folio,
-								folio_list))
+					if (data_race(!list_empty(&folio->_deferred_list)) &&
+					    split_folio_to_list(folio, folio_list))
 						goto activate_locked;
 				}
 				if (!add_to_swap(folio)) {
 					if (!folio_test_large(folio))
 						goto activate_locked_split;
 					/* Fallback to swap normal pages */
-					if (split_folio_to_list(folio,
-								folio_list))
+					if (split_folio_to_list(folio, folio_list))
 						goto activate_locked;
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-					count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
-					count_vm_event(THP_SWPOUT_FALLBACK);
+					if (nr_pages >= HPAGE_PMD_NR) {
+						count_memcg_folio_events(folio,
+							THP_SWPOUT_FALLBACK, 1);
+						count_vm_event(THP_SWPOUT_FALLBACK);
+					}
 #endif
 					if (!add_to_swap(folio))
 						goto activate_locked_split;