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 17:41:42 +0000
From:   John Garry <john.g.garry@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     axboe@...nel.dk, kbusch@...nel.org, 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 09/11/2023 15:13, Christoph Hellwig wrote:
> 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.

Right, FWIW we are only concerned with codepaths which use BIO_MAX_VECS, 
but I suppose that is not good enough as a guarantee.

> 
>> +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?

I think that this can be dropped as a comment. My earlier series used 
PAGE_SIZE and not sector size here, which I think was proper.

> And how would a type of userspace I/O even matter for low-level
> block code.

It shouldn't do, but we still need to limit according to request queue 
limits.

>  What if I wanted to use this for file system metadata?
> 

As mentioned, I think that the direct-IO comment can be dropped.

Thanks,
John