[PATCH v2 1/2] mm:vmscan: the dirty folio in folio list skip unmap

[Zhiguo Jiang]

In the shrink_folio_list() the sources of the file dirty folio include
two ways below:
1. The dirty folio is from the incoming parameter folio_list,
   which is the inactive file lru.
2. The dirty folio is from the PTE dirty bit transferred by
   the try_to_unmap().

For the first source of the dirty folio, if the dirty folio does not
support pageout, the dirty folio can skip unmap in advance to reduce
recyling time.

Signed-off-by: Zhiguo Jiang <justinjiang@xxxxxxxx>
---

Changelog:
v1->v2:
1. Keep the original judgment flow.
2. Add the interface of folio_check_pageout().
3. The dirty folio which does not support pageout in inactive file lru
   skip unmap in advance.

 mm/vmscan.c | 103 +++++++++++++++++++++++++++++++++-------------------
 1 file changed, 66 insertions(+), 37 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a68d01fcc307..e067269275a5 100755
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -925,6 +925,44 @@ static void folio_check_dirty_writeback(struct folio *folio,
 		mapping->a_ops->is_dirty_writeback(folio, dirty, writeback);
 }
 
+/* Check if a dirty folio can support pageout in the recyling process*/
+static bool folio_check_pageout(struct folio *folio,
+						struct pglist_data *pgdat)
+{
+	int ret = true;
+
+	/*
+	 * Anonymous folios are not handled by flushers and must be written
+	 * from reclaim context. Do not stall reclaim based on them.
+	 * MADV_FREE anonymous folios are put into inactive file list too.
+	 * They could be mistakenly treated as file lru. So further anon
+	 * test is needed.
+	 */
+	if (!folio_is_file_lru(folio) ||
+		(folio_test_anon(folio) && !folio_test_swapbacked(folio)))
+		goto out;
+
+	if (folio_test_dirty(folio) &&
+		(!current_is_kswapd() ||
+		 !folio_test_reclaim(folio) ||
+		 !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
+		/*
+		 * Immediately reclaim when written back.
+		 * Similar in principle to folio_deactivate()
+		 * except we already have the folio isolated
+		 * and know it's dirty
+		 */
+		node_stat_mod_folio(folio, NR_VMSCAN_IMMEDIATE,
+			folio_nr_pages(folio));
+		folio_set_reclaim(folio);
+
+		ret = false;
+	}
+
+out:
+	return ret;
+}
+
 static struct folio *alloc_demote_folio(struct folio *src,
 		unsigned long private)
 {
@@ -1078,6 +1116,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		if (dirty && !writeback)
 			stat->nr_unqueued_dirty += nr_pages;
 
+		/* If the dirty folio dose not support pageout,
+		 * the dirty folio can skip this recycling.
+		 */
+		if (!folio_check_pageout(folio, pgdat))
+			goto activate_locked;
+
 		/*
 		 * Treat this folio as congested if folios are cycling
 		 * through the LRU so quickly that the folios marked
@@ -1261,43 +1305,6 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 			enum ttu_flags flags = TTU_BATCH_FLUSH;
 			bool was_swapbacked = folio_test_swapbacked(folio);
 
-			if (folio_test_dirty(folio)) {
-				/*
-				 * Only kswapd can writeback filesystem folios
-				 * to avoid risk of stack overflow. But avoid
-				 * injecting inefficient single-folio I/O into
-				 * flusher writeback as much as possible: only
-				 * write folios when we've encountered many
-				 * dirty folios, and when we've already scanned
-				 * the rest of the LRU for clean folios and see
-				 * the same dirty folios again (with the reclaim
-				 * flag set).
-				 */
-				if (folio_is_file_lru(folio) &&
-					(!current_is_kswapd() ||
-					 !folio_test_reclaim(folio) ||
-					 !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
-					/*
-					 * Immediately reclaim when written back.
-					 * Similar in principle to folio_deactivate()
-					 * except we already have the folio isolated
-					 * and know it's dirty
-					 */
-					node_stat_mod_folio(folio, NR_VMSCAN_IMMEDIATE,
-							nr_pages);
-					folio_set_reclaim(folio);
-
-					goto activate_locked;
-				}
-
-				if (references == FOLIOREF_RECLAIM_CLEAN)
-					goto keep_locked;
-				if (!may_enter_fs(folio, sc->gfp_mask))
-					goto keep_locked;
-				if (!sc->may_writepage)
-					goto keep_locked;
-			}
-
 			if (folio_test_pmd_mappable(folio))
 				flags |= TTU_SPLIT_HUGE_PMD;
 
@@ -1323,6 +1330,28 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 
 		mapping = folio_mapping(folio);
 		if (folio_test_dirty(folio)) {
+			/*
+			 * Only kswapd can writeback filesystem folios
+			 * to avoid risk of stack overflow. But avoid
+			 * injecting inefficient single-folio I/O into
+			 * flusher writeback as much as possible: only
+			 * write folios when we've encountered many
+			 * dirty folios, and when we've already scanned
+			 * the rest of the LRU for clean folios and see
+			 * the same dirty folios again (with the reclaim
+			 * flag set).
+			 */
+			if (folio_is_file_lru(folio) &&
+				!folio_check_pageout(folio, pgdat))
+				goto activate_locked;
+
+			if (references == FOLIOREF_RECLAIM_CLEAN)
+				goto keep_locked;
+			if (!may_enter_fs(folio, sc->gfp_mask))
+				goto keep_locked;
+			if (!sc->may_writepage)
+				goto keep_locked;
+
 			/*
 			 * Folio is dirty. Flush the TLB if a writable entry
 			 * potentially exists to avoid CPU writes after I/O
-- 
2.39.0