[PATCH 2/2] md: Fix forward incompatibility from configurable logical block size

From: Li Nan <[email protected]>

Commit 62ed1b582246 ("md: allow configuring logical block size") used
reserved pad to add 'logical_block_size' to metadata. RAID rejects
non-zero reserved pad, so arrays fail when rolling back to old kernels
after booting new ones.

Set 'logical_block_size' only for newly created arrays to support rollback
to old kernels. Importantly new arrays still won't work on old kernels to
prevent data loss issue from LBS changes.

For arrays created on old kernels which confirmed not to rollback,
configure LBS by echo 'enable' to md/logical_block_size.

Fixes: 62ed1b582246 ("md: allow configuring logical block size")
Reported-by: BugReports <[email protected]>
Closes: https://lore.kernel.org/linux-raid/[email protected]/T/#t
Signed-off-by: Li Nan <[email protected]>
---
 drivers/md/md.c | 44 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 7c0dd94a4d25..28c9435016fe 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2014,8 +2014,14 @@ static int super_1_validate(struct mddev *mddev, struct md_rdev *freshest, struc
 
 		mddev->max_disks =  (4096-256)/2;
 
-		if (!mddev->logical_block_size)
+		if (!mddev->logical_block_size) {
 			mddev->logical_block_size = le32_to_cpu(sb->logical_block_size);
+			if (!mddev->logical_block_size)
+				pr_warn("%s: echo 'enable' to md/logical_block_size to prevent data loss issue from LBS changes.\n"
+					"    Note: After enable, array will not be assembled in old kernels (<= 6.18)\n",
+					mdname(mddev));
+		}
+
 
 		if ((le32_to_cpu(sb->feature_map) & MD_FEATURE_BITMAP_OFFSET) &&
 		    mddev->bitmap_info.file == NULL) {
@@ -5983,8 +5989,27 @@ lbs_store(struct mddev *mddev, const char *buf, size_t len)
 	if (mddev->major_version == 0)
 		return -EINVAL;
 
-	if (mddev->pers)
-		return -EBUSY;
+	if (mddev->pers) {
+		if (mddev->logical_block_size)
+			return -EBUSY;
+		/*
+		 * To fix forward compatibility issues, LBS is not
+		 * configured in old kernels array (<=6.18) by default.
+		 * If the user confirms no rollback to old kernels,
+		 * enable LBS by writing "enable" — to prevent data
+		 * loss from LBS changes.
+		 */
+		if (cmd_match(buf, "enable")) {
+			mddev->logical_block_size =
+				queue_logical_block_size(mddev->gendisk->queue);
+			set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
+			pr_info("%s: config logical block size success, array will not be assembled in old kernels (<= 6.18)\n",
+				mdname(mddev));
+			return len;
+		} else {
+			return -EBUSY;
+		}
+	}
 
 	err = kstrtouint(buf, 10, &lbs);
 	if (err < 0)
@@ -6165,7 +6190,18 @@ int mddev_stack_rdev_limits(struct mddev *mddev, struct queue_limits *lim,
 			mdname(mddev));
 		return -EINVAL;
 	}
-	mddev->logical_block_size = lim->logical_block_size;
+
+	/*
+	 * Fix forward compatibility issue. Only set LBS by default for
+	 * new array, mddev->events == 0 indicates the array was just
+	 * created. When assembling an array, read LBS from the superblock
+	 * instead — LBS is 0 in superblocks created by old kernels.
+	 */
+	if (!mddev->events && mddev->major_version == 1) {
+		pr_info("%s: array will not be assembled in old kernels that lack configurable lbs support (<= 6.18)\n",
+			mdname(mddev));
+		mddev->logical_block_size = lim->logical_block_size;
+	}
 
 	return 0;
 }
-- 
2.39.2