linux-kernel - Re: [PATCH] dm-table: Set BLK_FEAT_ATOMIC

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Message-ID: <57c7b277-4afd-cc22-d1aa-90ad2d9d23ea@redhat.com>
Date: Sun, 4 May 2025 12:05:58 +0200 (CEST)
From: Mikulas Patocka <mpatocka@...hat.com>
To: John Garry <john.g.garry@...cle.com>
cc: agk@...hat.com, snitzer@...nel.org, dm-devel@...ts.linux.dev, 
    linux-kernel@...r.kernel.org, martin.petersen@...cle.com
Subject: Re: [PATCH] dm-table: Set BLK_FEAT_ATOMIC_WRITES for target queue
 limits

Hi

I applied the patch, thanks.

Mikulas



On Fri, 2 May 2025, John Garry wrote:

> Feature flag BLK_FEAT_ATOMIC_WRITES is not being properly set for the
> target queue limits, and this means that atomic writes are not being
> enabled for any dm personalities.
> 
> When calling dm_set_device_limits() -> blk_stack_limits() ->
> ... -> blk_stack_atomic_writes_limits(), the bottom device limits
> (which corresponds to intermediate target queue limits) does not have
> BLK_FEAT_ATOMIC_WRITES set, and so atomic writes can never be enabled.
> 
> Typically such a flag would be inherited from the stacked device in
> dm_set_device_limits() -> blk_stack_limits() via BLK_FEAT_INHERIT_MASK,
> but BLK_FEAT_ATOMIC_WRITES is not inherited as it's preferred to manually
> enable on a per-personality basis.
> 
> Set BLK_FEAT_ATOMIC_WRITES manually for the intermediate target queue
> limits from the stacked device to get atomic writes working.
> 
> Fixes: 3194e36488e2 ("dm-table: atomic writes support")
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 9e175c5e0634..b35e48fed641 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -431,6 +431,12 @@ static int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
>  		return 0;
>  	}
>  
> +	/*
> +	 * BLK_FEAT_ATOMIC_WRITES is not inherited from the bottom device in
> +	 * blk_stack_limits(), so do it manually.
> +	 */
> +	limits->features |= (q->limits.features & BLK_FEAT_ATOMIC_WRITES);
> +
>  	if (blk_stack_limits(limits, &q->limits,
>  			get_start_sect(bdev) + start) < 0)
>  		DMWARN("%s: adding target device %pg caused an alignment inconsistency: "
> -- 
> 2.31.1
>