linux-kernel - [PATCH] md/raid1: skip recovery of already synced areas

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Message-Id: <20250910082544.271923-1-linan666@huaweicloud.com>
Date: Wed, 10 Sep 2025 16:25:44 +0800
From: linan666@...weicloud.com
To: song@...nel.org,
	yukuai3@...wei.com
Cc: linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linan666@...weicloud.com,
	yangerkun@...wei.com,
	yi.zhang@...wei.com
Subject: [PATCH] md/raid1: skip recovery of already synced areas

From: Li Nan <linan122@...wei.com>

When a new disk is added during running recovery, the kernel may
restart recovery from the beginning of the device and submit write
io to ranges that have already been synchronized.

Reproduce:
  mdadm -CR /dev/md0 -l1 -n3 /dev/sda missing missing
  mdadm --add /dev/md0 /dev/sdb
  sleep 10
  cat /proc/mdstat	# partially synchronized
  mdadm --add /dev/md0 /dev/sdc
  cat /proc/mdstat	# start from 0
  iostat 1 sdb sdc	# sdb has io, too

If 'rdev->recovery_offset' is ahead of the current recovery sector,
read from that device instead of issuing a write. It prevents
unnecessary writes while still preserving the chance to back up data
if it is the last copy.

Signed-off-by: Li Nan <linan122@...wei.com>
---
 drivers/md/raid1.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 3e422854cafb..ac5a9b73157a 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -2894,7 +2894,8 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
 		    test_bit(Faulty, &rdev->flags)) {
 			if (i < conf->raid_disks)
 				still_degraded = true;
-		} else if (!test_bit(In_sync, &rdev->flags)) {
+		} else if (!test_bit(In_sync, &rdev->flags) &&
+			   rdev->recovery_offset <= sector_nr) {
 			bio->bi_opf = REQ_OP_WRITE;
 			bio->bi_end_io = end_sync_write;
 			write_targets ++;
@@ -2903,6 +2904,9 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
 			sector_t first_bad = MaxSector;
 			sector_t bad_sectors;
 
+			if (!test_bit(In_sync, &rdev->flags))
+				good_sectors = min(rdev->recovery_offset - sector_nr,
+						   (u64)good_sectors);
 			if (is_badblock(rdev, sector_nr, good_sectors,
 					&first_bad, &bad_sectors)) {
 				if (first_bad > sector_nr)
-- 
2.39.2