linux-kernel - Re: [PATCH 17/21] fs: xfs: iomap atomic write support

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Date:   Tue, 28 Nov 2023 14:56:19 +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
Subject: Re: [PATCH 17/21] fs: xfs: iomap atomic write support

On Tue, Nov 28, 2023 at 08:56:37AM +0000, John Garry wrote:
> Are you suggesting some sort of hybrid between the atomic write series you 
> had a few years ago and this solution?

Very roughly, yes.

> To me that would be continuing with the following:
> - per-IO RWF_ATOMIC (and not O_ATOMIC semantics of nothing is written until 
> some data sync)

Yes.

> - writes must be a power-of-two and at a naturally-aligned offset

Where offset is offset in the file?  It would not require it.  You
probably want to do it for optimal performance, but requiring it
feeels rather limited.

> - relying on atomic write HW support always

And I think that's where we have different opinions.  I think the hw
offload is a nice optimization and we should use it wherever we can.
But building the entire userspace API around it feels like a mistake.

> BTW, we also have rtvol support which does not use forcealign as it already 
> can guarantee alignment, but still does rely on the same principle of 
> requiring alignment - would you want CoW support there also?

Upstream doesn't have out of place write support for the RT subvolume
yet.  But Darrick has a series for it and we're actively working on
upstreaming it.