linux-kernel - Re: [PATCH v2 1/3] md/raid1,raid10: don't broken array on failfast metadata write fails

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Message-ID: <725043ad-2d50-be78-7cc3-8c565ab364e0@huaweicloud.com>
Date: Sat, 23 Aug 2025 09:54:11 +0800
From: Li Nan <linan666@...weicloud.com>
To: Kenta Akagi <k@...l.me>, Song Liu <song@...nel.org>,
 Yu Kuai <yukuai3@...wei.com>, Mariusz Tkaczyk <mtkaczyk@...nel.org>,
 Guoqing Jiang <jgq516@...il.com>
Cc: linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] md/raid1,raid10: don't broken array on failfast
 metadata write fails



在 2025/8/18 1:27, Kenta Akagi 写道:
> A super_write IO failure with MD_FAILFAST must not cause the array
> to fail.
> 
> Because a failfast bio may fail even when the rdev is not broken,
> so IO must be retried rather than failing the array when a metadata
> write with MD_FAILFAST fails on the last rdev.
> 
> A metadata write with MD_FAILFAST is retried after failure as
> follows:
> 
> 1. In super_written, MD_SB_NEED_REWRITE is set in sb_flags.
> 
> 2. In md_super_wait, which is called by the function that
> executed md_super_write and waits for completion,
> -EAGAIN is returned because MD_SB_NEED_REWRITE is set.
> 
> 3. The caller of md_super_wait (such as md_update_sb)
> receives a negative return value and then retries md_super_write.
> 
> 4. The md_super_write function, which is called to perform
> the same metadata write, issues a write bio without MD_FAILFAST
> this time.
> 
> When a write from super_written without MD_FAILFAST fails,
> the array may broken, and MD_BROKEN should be set.
> 
> After commit 9631abdbf406 ("md: Set MD_BROKEN for RAID1 and RAID10"),
> calling md_error on the last rdev in RAID1/10 always sets
> the MD_BROKEN flag on the array.
> As a result, when failfast IO fails on the last rdev, the array
> immediately becomes failed.
> 
> This commit prevents MD_BROKEN from being set when a super_write with
> MD_FAILFAST fails on the last rdev, ensuring that the array does
> not become failed due to failfast IO failures.
> 
> Failfast IO failures on any rdev except the last one are not retried
> and are marked as Faulty immediately. This minimizes array IO latency
> when an rdev fails.
> 
> Fixes: 9631abdbf406 ("md: Set MD_BROKEN for RAID1 and RAID10")
> Signed-off-by: Kenta Akagi <k@...l.me>


[...]

> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -1746,8 +1746,12 @@ static void raid1_status(struct seq_file *seq, struct mddev *mddev)
>    *	- recovery is interrupted.
>    *	- &mddev->degraded is bumped.
>    *
> - * @rdev is marked as &Faulty excluding case when array is failed and
> - * &mddev->fail_last_dev is off.
> + * If @rdev is marked with &FailfastIOFailure, it means that super_write
> + * failed in failfast and will be retried, so the @mddev did not fail.
> + *
> + * @rdev is marked as &Faulty excluding any cases:
> + *	- when @mddev is failed and &mddev->fail_last_dev is off
> + *	- when @rdev is last device and &FailfastIOFailure flag is set
>    */
>   static void raid1_error(struct mddev *mddev, struct md_rdev *rdev)
>   {
> @@ -1758,6 +1762,10 @@ static void raid1_error(struct mddev *mddev, struct md_rdev *rdev)
>   
>   	if (test_bit(In_sync, &rdev->flags) &&
>   	    (conf->raid_disks - mddev->degraded) == 1) {
> +		if (test_bit(FailfastIOFailure, &rdev->flags)) {
> +			spin_unlock_irqrestore(&conf->device_lock, flags);
> +			return;
> +		}
>   		set_bit(MD_BROKEN, &mddev->flags);
>   
>   		if (!mddev->fail_last_dev) {

At this point, users who try to fail this rdev will get a successful return
without Faulty flag. Should we consider it?

-- 
Thanks,
Nan