linux-kernel - Re: [Patch v9 05/10] block: Add core atomic write support

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Date: Fri, 21 Jun 2024 08:09:46 +0200
From: Hannes Reinecke <hare@...e.de>
To: 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, viro@...iv.linux.org.uk, brauner@...nel.org,
 dchinner@...hat.com, jack@...e.cz
Cc: djwong@...nel.org, 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, willy@...radead.org, agk@...hat.com,
 snitzer@...nel.org, mpatocka@...hat.com, dm-devel@...ts.linux.dev,
 Himanshu Madhani <himanshu.madhani@...cle.com>
Subject: Re: [Patch v9 05/10] block: Add core atomic write support

On 6/20/24 14:53, John Garry wrote:
[ .. ]
> +/*
> + * Returns max guaranteed bytes which we can fit in a bio.
> + *
> + * We request that an atomic_write is ITER_UBUF iov_iter (so a single vector),
> + * so we assume that we can fit in at least PAGE_SIZE in a segment, apart from
> + * the first and last segments.
> + */
> +static
> +unsigned int blk_queue_max_guaranteed_bio(struct queue_limits *lim)
> +{
> +	unsigned int max_segments = min(BIO_MAX_VECS, lim->max_segments);
> +	unsigned int length;
> +
> +	length = min(max_segments, 2) * lim->logical_block_size;
> +	if (max_segments > 2)
> +		length += (max_segments - 2) * PAGE_SIZE;
> +
> +	return length;
> +}
> +
Now you got me confused.

Why is the length of an atomic write two times the logical block size?
And even if it does, shouldn't an atomic write be aligned to the logical 
block size, so why would you need to add two additional PAGE_SIZE worth
of length?
And even if _that_ would be okay, why PAGE_SIZE? We're trying really 
hard to get away from implicit PAGE_SIZE assumptions when doing I/O ...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich