linux-ext4 - Re: [RFC PATCH 0/8] xfs: single block atomic writes for buffered IO

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Message-ID: <aRWzq_LpoJHwfYli@dread.disaster.area>
Date: Thu, 13 Nov 2025 21:32:11 +1100
From: Dave Chinner <david@...morbit.com>
To: Ritesh Harjani <ritesh.list@...il.com>
Cc: Christoph Hellwig <hch@....de>, Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
	Christian Brauner <brauner@...nel.org>, djwong@...nel.org,
	john.g.garry@...cle.com, tytso@....edu, willy@...radead.org,
	dchinner@...hat.com, linux-xfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, jack@...e.cz,
	nilay@...ux.ibm.com, martin.petersen@...cle.com,
	rostedt@...dmis.org, axboe@...nel.dk, linux-block@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/8] xfs: single block atomic writes for buffered IO

On Thu, Nov 13, 2025 at 11:12:49AM +0530, Ritesh Harjani wrote:
> Christoph Hellwig <hch@....de> writes:
> 
> > On Thu, Nov 13, 2025 at 08:56:56AM +1100, Dave Chinner wrote:
> >> On Wed, Nov 12, 2025 at 04:36:03PM +0530, Ojaswin Mujoo wrote:
> >> > This patch adds support to perform single block RWF_ATOMIC writes for
> >> > iomap xfs buffered IO. This builds upon the inital RFC shared by John
> >> > Garry last year [1]. Most of the details are present in the respective 
> >> > commit messages but I'd mention some of the design points below:
> >> 
> >> What is the use case for this functionality? i.e. what is the
> >> reason for adding all this complexity?
> >
> > Seconded.  The atomic code has a lot of complexity, and further mixing
> > it with buffered I/O makes this even worse.  We'd need a really important
> > use case to even consider it.
> 
> I agree this should have been in the cover letter itself. 
> 
> I believe the reason for adding this functionality was also discussed at
> LSFMM too...  
> 
> For e.g. https://lwn.net/Articles/974578/ goes in depth and talks about
> Postgres folks looking for this, since PostgreSQL databases uses
> buffered I/O for their database writes.

Pointing at a discussion about how "this application has some ideas
on how it can maybe use it someday in the future" isn't a
particularly good justification. This still sounds more like a
research project than something a production system needs right now.

Why didn't you use the existing COW buffered write IO path to
implement atomic semantics for buffered writes? The XFS
functionality is already all there, and it doesn't require any
changes to the page cache or iomap to support...

-Dave.
-- 
Dave Chinner
david@...morbit.com