[PATCH] f2fs: issue multi-device flushes in parallel

[Yonggil Song <yonggil.song@samsung.com>]

On a multi-device setup, submit_flush_wait() walked the dirty devices
in order and aborted the whole loop on the first device whose flush
failed, leaving the remaining dirty devices un-flushed. Each device
still needs its own data made durable, so a failure on one device must
not skip the others. It also waited for one device's flush to complete
before issuing the next, even though the devices have independent
flush queues and could be flushed concurrently.

Flush every dirty device best-effort and in parallel instead: build
one PREFLUSH bio per dirty device, submit them all, then wait for
every completion, returning the first error seen (0 if all succeed).
This bounds the flush window by the slowest device rather than the sum
of all of them. No caller depends on the previous early-abort
behaviour -- fsync only checks whether the return value is zero
(fs/f2fs/file.c). The checkpoint path (f2fs_flush_device_cache) is
unaffected; this only touches the fsync flush path.

Signed-off-by: Yonggil Song <yonggil.song@samsung.com>
---
 fs/f2fs/segment.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 78 insertions(+), 6 deletions(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index d71ddb3ee918..51d7f76e3d1d 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -566,24 +566,96 @@ static int __submit_flush_wait(struct f2fs_sb_info *sbi,
 	return ret;
 }
 
-static int submit_flush_wait(struct f2fs_sb_info *sbi, nid_t ino)
+static void f2fs_flush_end_io(struct bio *bio)
+{
+	complete(bio->bi_private);
+}
+
+struct f2fs_flush_bio {
+	struct bio bio;
+	struct completion wait;
+};
+
+/*
+ * Flush every dirty device best-effort: a failure on one device must not
+ * skip the flush on the remaining dirty devices, since each device still
+ * needs its own data made durable. Report the first error.
+ */
+static int submit_flush_wait_serial(struct f2fs_sb_info *sbi, nid_t ino)
 {
 	int ret = 0;
 	int i;
 
-	if (!f2fs_is_multi_device(sbi))
-		return __submit_flush_wait(sbi, sbi->sb->s_bdev);
+	for (i = 0; i < sbi->s_ndevs; i++) {
+		int err;
+
+		if (!f2fs_is_dirty_device(sbi, ino, i, FLUSH_INO))
+			continue;
+		err = __submit_flush_wait(sbi, FDEV(i).bdev);
+		if (err && !ret)
+			ret = err;
+	}
+	return ret;
+}
+
+/*
+ * Same best-effort/first-error contract as submit_flush_wait_serial(), but
+ * issue every dirty device's flush before waiting for any of them, so the
+ * per-device flush latencies overlap instead of adding up in series. Fall
+ * back to the serial path if the bio array cannot be allocated.
+ */
+static int submit_flush_wait_parallel(struct f2fs_sb_info *sbi, nid_t ino)
+{
+	struct f2fs_flush_bio *flush_bio;
+	unsigned long devices = 0;
+	int ret = 0;
+	int i;
+
+	flush_bio = kcalloc(sbi->s_ndevs, sizeof(*flush_bio), GFP_NOFS);
+	if (!flush_bio)
+		return submit_flush_wait_serial(sbi, ino);
 
 	for (i = 0; i < sbi->s_ndevs; i++) {
 		if (!f2fs_is_dirty_device(sbi, ino, i, FLUSH_INO))
 			continue;
-		ret = __submit_flush_wait(sbi, FDEV(i).bdev);
-		if (ret)
-			break;
+
+		bio_init(&flush_bio[i].bio, FDEV(i).bdev, NULL, 0,
+			 REQ_OP_WRITE | REQ_PREFLUSH);
+		init_completion(&flush_bio[i].wait);
+		flush_bio[i].bio.bi_private = &flush_bio[i].wait;
+		flush_bio[i].bio.bi_end_io = f2fs_flush_end_io;
+		submit_bio(&flush_bio[i].bio);
+		devices |= BIT(i);
+	}
+
+	for (i = 0; i < sbi->s_ndevs; i++) {
+		int err;
+
+		if (!(devices & BIT(i)))
+			continue;
+
+		wait_for_completion(&flush_bio[i].wait);
+		err = blk_status_to_errno(flush_bio[i].bio.bi_status);
+		trace_f2fs_issue_flush(FDEV(i).bdev, test_opt(sbi, NOBARRIER),
+				       test_opt(sbi, FLUSH_MERGE), err);
+		if (!err)
+			f2fs_update_iostat(sbi, NULL, FS_FLUSH_IO, 0);
+		else if (!ret)
+			ret = err;
+		bio_uninit(&flush_bio[i].bio);
 	}
+	kfree(flush_bio);
 	return ret;
 }
 
+static int submit_flush_wait(struct f2fs_sb_info *sbi, nid_t ino)
+{
+	if (!f2fs_is_multi_device(sbi))
+		return __submit_flush_wait(sbi, sbi->sb->s_bdev);
+
+	return submit_flush_wait_parallel(sbi, ino);
+}
+
 static int issue_flush_thread(void *data)
 {
 	struct f2fs_sb_info *sbi = data;
-- 
2.43.0