md: ensure child flush IO does not affect origin bio->bi_status

When a flush is issued to an RAID array, a child flush IO is created and
issued for each member disk in the RAID array. Since commit b75197e
("md: Remove flush handling"), each child flush IO has been chained with
the original bio. As a result, the failure of any child IO could modify
the bi_status of the original bio, potentially impacting the upper-layer
filesystem.

Fix the issue by preventing child flush IO from altering the original
bio->bi_status as before. However, this design introduces a known
issue: in the event of a power failure, if a flush IO on a member
disk fails, the upper layers may not be informed. This issue is not easy
to fix and will not be addressed for the time being in this issue.

Fixes: b75197e ("md: Remove flush handling")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240919063048.2887579-1-linan666@huaweicloud.com
Signed-off-by: Song Liu <song@kernel.org>

Author: Li Nan

drivers/md/md.c

@@ -546,6 +546,26 @@ static int mddev_set_closing_and_sync_blockdev(struct mddev *mddev, int opener_n
 	return 0;
 }
 
+/*
+ * The only difference from bio_chain_endio() is that the current
+ * bi_status of bio does not affect the bi_status of parent.
+ */
+static void md_end_flush(struct bio *bio)
+{
+	struct bio *parent = bio->bi_private;
+
+	/*
+	 * If any flush io error before the power failure,
+	 * disk data may be lost.
+	 */
+	if (bio->bi_status)
+		pr_err("md: %pg flush io error %d\n", bio->bi_bdev,
+			blk_status_to_errno(bio->bi_status));
+
+	bio_put(bio);
+	bio_endio(parent);
+}
+
 bool md_flush_request(struct mddev *mddev, struct bio *bio)
 {
 	struct md_rdev *rdev;
@@ -565,7 +585,9 @@ bool md_flush_request(struct mddev *mddev, struct bio *bio)
 		new = bio_alloc_bioset(rdev->bdev, 0,
 				       REQ_OP_WRITE | REQ_PREFLUSH, GFP_NOIO,
 				       &mddev->bio_set);
-		bio_chain(new, bio);
+		new->bi_private = bio;
+		new->bi_end_io = md_end_flush;
+		bio_inc_remaining(bio);
 		submit_bio(new);
 	}