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Date: Mon, 20 May 2024 11:47:54 -0400
From: Mike Snitzer <snitzer@kernel.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Theodore Ts'o <tytso@mit.edu>, dm-devel@lists.linux.dev,
	fstests@vger.kernel.org, linux-ext4@vger.kernel.org,
	regressions@lists.linux.dev, linux-block@vger.kernel.org
Subject: Re: dm: use queue_limits_set
Message-ID: <Zktwqu-N0E1miesx@kernel.org>
References: <20240518022646.GA450709@mit.edu>
 <ZkmIpCRaZE0237OH@kernel.org>
 <ZkmRKPfPeX3c138f@kernel.org>
 <20240520150653.GA32461@lst.de>
 <ZktuojMrQWH9MQJO@kernel.org>
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On Mon, May 20, 2024 at 11:39:14AM -0400, Mike Snitzer wrote:
> On Mon, May 20, 2024 at 05:06:53PM +0200, Christoph Hellwig wrote:
> 
> > This is probably my fault, I actually found this right at the time
> > of the original revert of switching dm to the limits API, and then
> > let it slip as the patch was reverted.  That fact that you readded
> > the commit somehow went past my attention window.
> 
> It's fine, all we can do now is work through how best to fix it.  Open
> to suggestions.  But this next hunk, which you trimmed in your reply,
> _seems_ needed to actually fix the issue Ted reported -- given the
> current validate method in blk-settings.c (resharing here to just
> continue this thread in a natural way):
> 
> diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
> index 4793ad2aa1f7..c196f39579af 100644
> --- a/drivers/md/dm-thin.c
> +++ b/drivers/md/dm-thin.c
> @@ -4497,7 +4499,8 @@ static void thin_io_hints(struct dm_target *ti, struct queue_limits *limits)
>  
>  	if (pool->pf.discard_enabled) {
>  		limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
> -		limits->max_discard_sectors = pool->sectors_per_block * BIO_PRISON_MAX_RANGE;
> +		limits->max_hw_discard_sectors = limits->max_user_discard_sectors =
> +			pool->sectors_per_block * BIO_PRISON_MAX_RANGE;
>  	}
>  }
>  
> 

Maybe update blk_validate_limits() to ensure max_discard_sectors is a
factor of discard_granularity?

That way thin_io_hints() (and equivalent functions in other DM
targets) just need to be audited/updated to ensure they are setting
both discard_granularity and max_hw_discard_sectors?

Mike