linux-kernel - Re: [PATCH v4 05/11] block: Add core atomic write support

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Date: Tue, 20 Feb 2024 09:22:45 +0100
From: Christoph Hellwig <hch@....de>
To: Dave Chinner <david@...morbit.com>
Cc: John Garry <john.g.garry@...cle.com>, axboe@...nel.dk,
	kbusch@...nel.org, hch@....de, sagi@...mberg.me, jejb@...ux.ibm.com,
	martin.petersen@...cle.com, djwong@...nel.org,
	viro@...iv.linux.org.uk, brauner@...nel.org, dchinner@...hat.com,
	jack@...e.cz, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
	linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
	linux-scsi@...r.kernel.org, ojaswin@...ux.ibm.com,
	linux-aio@...ck.org, linux-btrfs@...r.kernel.org,
	io-uring@...r.kernel.org, nilay@...ux.ibm.com,
	ritesh.list@...il.com
Subject: Re: [PATCH v4 05/11] block: Add core atomic write support

On Tue, Feb 20, 2024 at 09:58:39AM +1100, Dave Chinner wrote:
> > +	lim->atomic_write_hw_max_sectors = 0;
> > +	lim->atomic_write_max_sectors = 0;
> > +	lim->atomic_write_hw_boundary_sectors = 0;
> > +	lim->atomic_write_hw_unit_min_sectors = 0;
> > +	lim->atomic_write_unit_min_sectors = 0;
> > +	lim->atomic_write_hw_unit_max_sectors = 0;
> > +	lim->atomic_write_unit_max_sectors = 0;
> >  }
> 
> Seems to me this function would do better to just
> 
> 	memset(lim, 0, sizeof(*lim));
> 
> and then set all the non-zero fields.

. which the caller already has done :)  In the block tree this
function looks completely different now and relies on the caller
provided zeroing.

> > +void blk_queue_atomic_write_max_bytes(struct request_queue *q,
> > +				      unsigned int bytes)
> > +{
> > +	q->limits.atomic_write_hw_max_sectors = bytes >> SECTOR_SHIFT;
> > +	blk_atomic_writes_update_limits(q);
> > +}
> > +EXPORT_SYMBOL(blk_queue_atomic_write_max_bytes);
> 
> Ok, so this can silently set a limit that is different to what the
> caller asked to have set?
> 
> How is the caller supposed to find this out if the smaller limit
> that was set is not compatible with their configuration?
> 
> i.e. shouldn't this return an error if the requested size cannot
> be set exactly as specified?

That's how the blk limits all work.  The driver provides the hardware
capabilities for a given value, and the block layer ensures it
works with other limits imposed by the block layer or other parts
of the device limits.