Re: [PATCH v4 4/9] md/raid1,raid10: Don't set MD_BROKEN on failfast bio failure

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/09/15 11:42, Kenta Akagi 写道:
> Failfast is a feature implemented only for RAID1 and RAID10. It instructs
> the block device providing the rdev to immediately return a bio error
> without retrying if any issue occurs. This allows quickly detaching a
> problematic rdev and minimizes IO latency.
> 
> Due to its nature, failfast bios can fail easily, and md must not mark
> an essential rdev as Faulty or set MD_BROKEN on the array just because
> a failfast bio failed.
> 
> When failfast was introduced, RAID1 and RAID10 were designed to continue
> operating normally even if md_error was called for the last rdev. However,
> with the introduction of MD_BROKEN in RAID1/RAID10
> in commit 9631abdbf406 ("md: Set MD_BROKEN for RAID1 and RAID10"), calling
> md_error for the last rdev now prevents further writes to the array.
> Despite this, the current failfast error handler still assumes that
> calling md_error will not break the array.
> 
> Normally, this is not an issue because MD_FAILFAST is not set when a bio
> is issued to the last rdev. However, if the array is not degraded and a
> bio with MD_FAILFAST has been issued, simultaneous failures could
> potentially break the array. This is unusual but can happen; for example,
> this can occur when using NVMe over TCP if all rdevs depend on
> a single Ethernet link.
> 
> In other words, this becomes a problem under the following conditions:
> Preconditions:
> * Failfast is enabled on all rdevs.
> * All rdevs are In_sync - This is a requirement for bio to be submit
>    with MD_FAILFAST.
> * At least one bio has been submitted but has not yet completed.
> 
> Trigger condition:
> * All underlying devices of the rdevs return an error for their failfast
>    bios.
> 
> Whether the bio is a read or a write makes little difference to the
> outcome.
> In the write case, md_error is invoked on each rdev through its bi_end_io
> handler.
> In the read case, losing the first rdev triggers a metadata
> update. Next, md_super_write, unlike raid1_write_request, issues the bio
> with MD_FAILFAST if the rdev supports it, causing the bio to fail
> immediately - Before this patchset, LastDev was set only by the failure
> path in super_written. Consequently, super_written calls md_error on the
> remaining rdev.
> 
> Prior to this commit, the following changes were introduced:
> * The helper function md_bio_failure_error() that skips the error handler
>    if a failfast bio targets the last rdev.
> * Serialization md_error() and md_bio_failure_error().
> * Setting the LastDev flag for rdevs that must not be lost.
> 
> This commit uses md_bio_failure_error() instead of md_error() for failfast
> bio failures, ensuring that failfast bios do not stop array operations.
> 
> Fixes: 9631abdbf406 ("md: Set MD_BROKEN for RAID1 and RAID10")
> Signed-off-by: Kenta Akagi <k@mgml.me>
> ---
>   drivers/md/md.c     |  5 +----
>   drivers/md/raid1.c  | 37 ++++++++++++++++++-------------------
>   drivers/md/raid10.c |  9 +++++----
>   3 files changed, 24 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 65fdd9bae8f4..65814bbe9bad 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -1004,11 +1004,8 @@ static void super_written(struct bio *bio)
>   	if (bio->bi_status) {
>   		pr_err("md: %s gets error=%d\n", __func__,
>   		       blk_status_to_errno(bio->bi_status));
> -		md_error(mddev, rdev);
> -		if (!test_bit(Faulty, &rdev->flags)
> -		    && (bio->bi_opf & MD_FAILFAST)) {
> +		if (!md_bio_failure_error(mddev, rdev, bio))
>   			set_bit(MD_SB_NEED_REWRITE, &mddev->sb_flags);
> -		}
>   	}
>   
>   	bio_put(bio);
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 32ad6b102ff7..8fff9dacc6e0 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -470,7 +470,7 @@ static void raid1_end_write_request(struct bio *bio)
>   		    (bio->bi_opf & MD_FAILFAST) &&
>   		    /* We never try FailFast to WriteMostly devices */
>   		    !test_bit(WriteMostly, &rdev->flags)) {
> -			md_error(r1_bio->mddev, rdev);
> +			md_bio_failure_error(r1_bio->mddev, rdev, bio);
>   		}

Can following check of faulty replaced with return value?
>   
>   		/*
> @@ -2178,8 +2178,7 @@ static int fix_sync_read_error(struct r1bio *r1_bio)
>   	if (test_bit(FailFast, &rdev->flags)) {
>   		/* Don't try recovering from here - just fail it
>   		 * ... unless it is the last working device of course */
> -		md_error(mddev, rdev);
> -		if (test_bit(Faulty, &rdev->flags))
> +		if (md_bio_failure_error(mddev, rdev, bio))
>   			/* Don't try to read from here, but make sure
>   			 * put_buf does it's thing
>   			 */
> @@ -2657,9 +2656,8 @@ static void handle_write_finished(struct r1conf *conf, struct r1bio *r1_bio)
>   static void handle_read_error(struct r1conf *conf, struct r1bio *r1_bio)
>   {
>   	struct mddev *mddev = conf->mddev;
> -	struct bio *bio;
> +	struct bio *bio, *updated_bio;
>   	struct md_rdev *rdev;
> -	sector_t sector;
>   
>   	clear_bit(R1BIO_ReadError, &r1_bio->state);
>   	/* we got a read error. Maybe the drive is bad.  Maybe just
> @@ -2672,29 +2670,30 @@ static void handle_read_error(struct r1conf *conf, struct r1bio *r1_bio)
>   	 */
>   
>   	bio = r1_bio->bios[r1_bio->read_disk];
> -	bio_put(bio);
> -	r1_bio->bios[r1_bio->read_disk] = NULL;
> +	updated_bio = NULL;
>   
>   	rdev = conf->mirrors[r1_bio->read_disk].rdev;
> -	if (mddev->ro == 0
> -	    && !test_bit(FailFast, &rdev->flags)) {
> -		freeze_array(conf, 1);
> -		fix_read_error(conf, r1_bio);
> -		unfreeze_array(conf);
> -	} else if (mddev->ro == 0 && test_bit(FailFast, &rdev->flags)) {
> -		md_error(mddev, rdev);
> +	if (mddev->ro == 0) {
> +		if (!test_bit(FailFast, &rdev->flags)) {
> +			freeze_array(conf, 1);
> +			fix_read_error(conf, r1_bio);
> +			unfreeze_array(conf);
> +		} else {
> +			md_bio_failure_error(mddev, rdev, bio);
> +		}
>   	} else {
> -		r1_bio->bios[r1_bio->read_disk] = IO_BLOCKED;
> +		updated_bio = IO_BLOCKED;
>   	}

I'll suggest a separate patch to cleanup the conditions first, it's
better for code review.

BTW, I'll prefer if else chain insted of nested if else, perhaps
following is better:

if (mddev->ro != 0) {
  /* read-only */
} else if (!test_bit(FailFast, &rdev->flags) {
  /* read-write and failfast is not set */
} else {
  /* read-write and failfast is set */
}
>   
> +	bio_put(bio);
> +	r1_bio->bios[r1_bio->read_disk] = updated_bio;
> +
>   	rdev_dec_pending(rdev, conf->mddev);
> -	sector = r1_bio->sector;
> -	bio = r1_bio->master_bio;
>   
>   	/* Reuse the old r1_bio so that the IO_BLOCKED settings are preserved */
>   	r1_bio->state = 0;
> -	raid1_read_request(mddev, bio, r1_bio->sectors, r1_bio);
> -	allow_barrier(conf, sector);
> +	raid1_read_request(mddev, r1_bio->master_bio, r1_bio->sectors, r1_bio);
> +	allow_barrier(conf, r1_bio->sector);
>   }
>   
>   static void raid1d(struct md_thread *thread)
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index dc4edd4689f8..b73af94a88b0 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -488,7 +488,7 @@ static void raid10_end_write_request(struct bio *bio)
>   			dec_rdev = 0;
>   			if (test_bit(FailFast, &rdev->flags) &&
>   			    (bio->bi_opf & MD_FAILFAST)) {
> -				md_error(rdev->mddev, rdev);
> +				md_bio_failure_error(rdev->mddev, rdev, bio);
>   			}
>  

Same as raid1, can following check of faulty replaced of return value.
>   			/*
> @@ -2443,7 +2443,7 @@ static void sync_request_write(struct mddev *mddev, struct r10bio *r10_bio)
>   				continue;
>   		} else if (test_bit(FailFast, &rdev->flags)) {
>   			/* Just give up on this device */
> -			md_error(rdev->mddev, rdev);
> +			md_bio_failure_error(rdev->mddev, rdev, tbio);
>   			continue;
>   		}
>   		/* Ok, we need to write this bio, either to correct an
> @@ -2895,8 +2895,9 @@ static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
>   		freeze_array(conf, 1);
>   		fix_read_error(conf, mddev, r10_bio);
>   		unfreeze_array(conf);
> -	} else
> -		md_error(mddev, rdev);
> +	} else {
> +		md_bio_failure_error(mddev, rdev, bio);
> +	}
>   
>   	rdev_dec_pending(rdev, mddev);
>   	r10_bio->state = 0;
> 

And please split this patch for raid1 and raid10.

Thanks
Kuai