linux-kernel - Re: [PATCH 1/4] iomap: Lift blocksize restriction on atomic writes

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Message-ID: <20250116065225.GA25695@lst.de>
Date: Thu, 16 Jan 2025 07:52:25 +0100
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Dave Chinner <david@...morbit.com>,
	John Garry <john.g.garry@...cle.com>, brauner@...nel.org,
	cem@...nel.org, dchinner@...hat.com, hch@....de,
	ritesh.list@...il.com, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	martin.petersen@...cle.com
Subject: Re: [PATCH 1/4] iomap: Lift blocksize restriction on atomic writes

On Tue, Jan 14, 2025 at 03:57:26PM -0800, Darrick J. Wong wrote:
> Ok, let's do that then.  Just to be clear -- for any RWF_ATOMIC direct
> write that's correctly aligned and targets a single mapping in the
> correct state, we can build the untorn bio and submit it.  For
> everything else, prealloc some post EOF blocks, write them there, and
> exchange-range them.
> 
> Tricky questions: How do we avoid collisions between overlapping writes?
> I guess we find a free file range at the top of the file that is long
> enough to stage the write, and put it there?  And purge it later?
> 
> Also, does this imply that the maximum file size is less than the usual
> 8EB?

I think literally using the exchrange code for anything but an
initial prototype is a bad idea for the above reasons.  If we go
beyond proving this is possible you'd want a version of exchrange
where the exchange partners is not a file mapping, but a cow staging
record.