linux-kernel - Re: [PATCH 01/21] block: Add atomic write operations to request

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Date:   Fri, 10 Nov 2023 07:23:25 +0100
From:   Christoph Hellwig <hch@....de>
To:     John Garry <john.g.garry@...cle.com>
Cc:     Christoph Hellwig <hch@....de>, 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,
        Himanshu Madhani <himanshu.madhani@...cle.com>
Subject: Re: [PATCH 01/21] block: Add atomic write operations to
 request_queue limits

On Thu, Nov 09, 2023 at 05:01:10PM +0000, John Garry wrote:
> Generally they come from the same device property. Then since 
> atomic_write_unit_max_bytes must be a power-of-2 (and 
> atomic_write_max_bytes may not be), they may be different.

How much do we care about supporting the additional slack over the
power of two version?  

> In addition, 
> atomic_write_unit_max_bytes is required to be limited by whatever is 
> guaranteed to be able to fit in a bio.

The limit what fits into a bio is UINT_MAX, not sure that matters :)

> atomic_write_max_bytes is really only relevant for merging writes. Maybe we 
> should not even expose via sysfs.

Or we need to have a good separate discussion on even supporting any
merges.  Willy chimed in that supporting merges was intentional,
but I'd really like to see numbers justifying it.