linux-kernel - [PATCH v2 4/4] migrate_pages: Don't wait forever locking buffers in MIGRATE_SYNC

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Message-ID: <20230421151135.v2.4.Ic39f0b16516acf4af1ce5d923150f93ee85a9398@changeid>
Date:   Fri, 21 Apr 2023 15:12:48 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>, Ying <ying.huang@...el.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Yu Zhao <yuzhao@...gle.com>, linux-fsdevel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>,
        Douglas Anderson <dianders@...omium.org>
Subject: [PATCH v2 4/4] migrate_pages: Don't wait forever locking buffers in MIGRATE_SYNC_LIGHT

Just as talked about in the patch ("migrate_pages: Don't wait forever
locking pages in MIGRATE_SYNC_LIGHT"), we don't really want unbounded
waits when we're running in MIGRATE_SYNC_LIGHT mode. Waiting on the
buffer lock is a second such unbounded wait. Let's put a timeout on
it.

While measurement didn't show this wait to be quite as bad as the one
waiting for the folio lock, it could still be measured to be over a
second in some cases.

Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

Changes in v2:
- "Don't wait forever locking buffers in MIGRATE_SYNC_LIGHT" new for v2.

 mm/migrate.c | 25 ++++++++-----------------
 1 file changed, 8 insertions(+), 17 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 60982df71a93..97c93604eb4c 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -715,25 +715,16 @@ static bool buffer_migrate_lock_buffers(struct buffer_head *head,
 							enum migrate_mode mode)
 {
 	struct buffer_head *bh = head;
+	bool locked;
 
-	/* Simple case, sync compaction */
-	if (mode != MIGRATE_ASYNC) {
-		do {
-			lock_buffer(bh);
-			bh = bh->b_this_page;
-
-		} while (bh != head);
-
-		return true;
-	}
-
-	/* async case, we cannot block on lock_buffer so use trylock_buffer */
 	do {
-		if (!trylock_buffer(bh)) {
-			/*
-			 * We failed to lock the buffer and cannot stall in
-			 * async migration. Release the taken locks
-			 */
+		if (mode == MIGRATE_ASYNC)
+			locked = trylock_buffer(bh);
+		else
+			locked = !lock_buffer_timeout(bh, timeout_for_mode(mode));
+
+		if (!locked) {
+			/* We failed to lock the buffer. Release the taken locks. */
 			struct buffer_head *failed_bh = bh;
 			bh = head;
 			while (bh != failed_bh) {
-- 
2.40.0.634.g4ca3ef3211-goog