linux-kernel - Re: [PATCH 02/21] block: Limit atomic writes according to bio and queue limits

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Date:   Thu, 9 Nov 2023 16:13:43 +0100
From:   Christoph Hellwig <hch@....de>
To:     John Garry <john.g.garry@...cle.com>
Cc:     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,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 02/21] block: Limit atomic writes according to bio and
 queue limits

On Fri, Sep 29, 2023 at 10:27:07AM +0000, John Garry wrote:
> We rely the block layer always being able to send a bio of size
> atomic_write_unit_max without being required to split it due to request
> queue or other bio limits.
> 
> A bio may contain min(BIO_MAX_VECS, limits->max_segments) vectors,
> and each vector is at worst case the device logical block size from
> direct IO alignment requirement.

A bio can have more than BIO_MAX_VECS if you use bio_init.

> +static unsigned int blk_queue_max_guaranteed_bio_size_sectors(
> +					struct request_queue *q)
> +{
> +	struct queue_limits *limits = &q->limits;
> +	unsigned int max_segments = min_t(unsigned int, BIO_MAX_VECS,
> +					limits->max_segments);
> +	/*  Limit according to dev sector size as we only support direct-io */

Who is "we", and how tells the caller to only ever use direct I/O?
And how would a type of userspace I/O even matter for low-level
block code.  What if I wanted to use this for file system metadata?