[PATCH v3 5/5] mm/vmscan: flush TLB for every 31 folios evictions

[Zhang Peng <zippermonkey@icloud.com>]

From: Zhang Peng <bruzzhang@tencent.com>

Currently we flush TLB for every dirty folio, which is a bottleneck for
systems with many cores as this causes heavy IPI usage.

So instead, batch the folios, and flush once for every 31 folios (one
folio_batch). These folios will be held in a folio_batch releasing their
lock, then when folio_batch is full, do following steps:

- For each folio: lock - check still evictable (writeback, mapped,
  dma_pinned)
  - If no longer evictable, put back to LRU
- Flush TLB once for the batch
- Pageout the folios

Suggested-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Zhang Peng <bruzzhang@tencent.com>
---
 mm/vmscan.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 68 insertions(+), 7 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 63cc88c875e8..8d18e1c5b53d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1217,6 +1217,55 @@ static void pageout_one(struct folio *folio, struct list_head *ret_folios,
 			folio_test_unevictable(folio), folio);
 }
 
+static void pageout_batch(struct folio_batch *fbatch,
+			  struct list_head *ret_folios,
+			  struct folio_batch *free_folios,
+			  struct scan_control *sc, struct reclaim_stat *stat,
+			  struct swap_iocb **plug, struct list_head *folio_list)
+{
+	int i, count = folio_batch_count(fbatch);
+	struct folio *folio;
+
+	/*
+	 * Reuse fbatch in-place: reinit only clears the count, the
+	 * underlying folios array is still accessible via saved count.
+	 * Filter and re-add valid folios back into the same batch.
+	 */
+	folio_batch_reinit(fbatch);
+	for (i = 0; i < count; ++i) {
+		folio = fbatch->folios[i];
+		if (!folio_trylock(folio)) {
+			list_add(&folio->lru, ret_folios);
+			continue;
+		}
+
+		VM_WARN_ON_FOLIO(folio_test_lru(folio), folio);
+
+		if (folio_test_writeback(folio) || folio_mapped(folio) ||
+		    folio_maybe_dma_pinned(folio)) {
+			folio_unlock(folio);
+			list_add(&folio->lru, ret_folios);
+			continue;
+		}
+
+		folio_batch_add(fbatch, folio);
+	}
+
+	i = 0;
+	count = folio_batch_count(fbatch);
+	if (!count)
+		return;
+	/* One TLB flush for the batch */
+	try_to_unmap_flush_dirty();
+	for (i = 0; i < count; ++i) {
+		folio = fbatch->folios[i];
+		pageout_one(folio, ret_folios, free_folios, sc, stat, plug,
+			    folio_list);
+	}
+	/* Clear the batch for the caller's next use */
+	folio_batch_reinit(fbatch);
+}
+
 static bool folio_try_unmap(struct folio *folio, struct reclaim_stat *stat,
 			    unsigned int nr_pages)
 {
@@ -1264,6 +1313,8 @@ static void shrink_folio_list(struct list_head *folio_list,
 		struct mem_cgroup *memcg)
 {
 	struct folio_batch free_folios;
+	struct folio_batch flush_folios;
+
 	LIST_HEAD(ret_folios);
 	LIST_HEAD(demote_folios);
 	unsigned int nr_demoted = 0;
@@ -1272,6 +1323,8 @@ static void shrink_folio_list(struct list_head *folio_list,
 	struct swap_iocb *plug = NULL;
 
 	folio_batch_init(&free_folios);
+	folio_batch_init(&flush_folios);
+
 	memset(stat, 0, sizeof(*stat));
 	cond_resched();
 	do_demote_pass = can_demote(pgdat->node_id, sc, memcg);
@@ -1565,15 +1618,19 @@ static void shrink_folio_list(struct list_head *folio_list,
 				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
-			 * starts and then write it out here.
+			 * Unlock while batching: holding the lock until the
+			 * batch fills would stall swap faults that find this
+			 * folio via swap cache lookup. pageout_batch() will
+			 * relock each folio and recheck its state before
+			 * writing it out.
 			 */
-			try_to_unmap_flush_dirty();
-			pageout_one(folio, &ret_folios, &free_folios, sc, stat,
-				&plug, folio_list);
+			folio_unlock(folio);
+			if (!folio_batch_add(&flush_folios, folio))
+				pageout_batch(&flush_folios,
+							&ret_folios, &free_folios,
+							sc, stat, &plug,
+							folio_list);
 			goto next;
 		}
 
@@ -1603,6 +1660,10 @@ static void shrink_folio_list(struct list_head *folio_list,
 next:
 		continue;
 	}
+	if (folio_batch_count(&flush_folios)) {
+		pageout_batch(&flush_folios, &ret_folios, &free_folios, sc,
+			      stat, &plug, folio_list);
+	}
 	/* 'folio_list' is always empty here */
 
 	/* Migrate folios selected for demotion */

-- 
2.43.7