linux-kernel - Re: [PATCH 2/2] md: Fix forward incompatibility from configurable logical block size

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Message-Id: <f4fba874-9ad7-4efd-9240-79c452494b8a@fnnas.com>
Date: Thu, 25 Dec 2025 16:39:30 +0800
From: "Yu Kuai" <yukuai@...as.com>
To: <linan666@...weicloud.com>, <song@...nel.org>, <xni@...hat.com>, 
	<linan122@...wei.com>
Cc: <linux-raid@...r.kernel.org>, <linux-kernel@...r.kernel.org>, 
	<yangerkun@...wei.com>, <yi.zhang@...wei.com>, <bugreports61@...il.com>, 
	<yukuai@...as.com>
Subject: Re: [PATCH 2/2] md: Fix forward incompatibility from configurable logical block size

Hi,

在 2025/12/19 17:21, linan666@...weicloud.com 写道:
> From: Li Nan <linan122@...wei.com>
>
> 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 <bugreports61@...il.com>
> Closes: https://lore.kernel.org/linux-raid/825e532d-d1e1-44bb-5581-692b7c091796@huaweicloud.com/T/#t
> Signed-off-by: Li Nan <linan122@...wei.com>
> ---
>   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));

Looks like this will print the same message for every rdev added to this array, please move the warning to
mddev_stack_rdev_limits() as well.

> +		}
> +
>   
>   		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));

Instead of a new string "enabled", I'll prefer to echo the exact lbs.

> +			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;
>   }

-- 
Thansk,
Kuai