linux-kernel - Re: [PATCH v2 2/7] iomap: Add zero unwritten mappings dio support

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Message-ID: <20241217070845.GA19358@lst.de>
Date: Tue, 17 Dec 2024 08:08:45 +0100
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@....de>, John Garry <john.g.garry@...cle.com>,
	brauner@...nel.org, cem@...nel.org, dchinner@...hat.com,
	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 v2 2/7] iomap: Add zero unwritten mappings dio support

On Fri, Dec 13, 2024 at 04:56:38PM -0800, Darrick J. Wong wrote:
> > > "If you receive -EBADMAP, then call fallocate(FALLOC_FL_MAKE_OVERWRITE)
> > > to force all the mappings to pure overwrites."
> > 
> > Ewwwwwwwwwwwwwwwwwwwww.
> > 
> > That's not a sane API in any way.
> 
> Oh I know, I'd much rather stick to the view that block untorn writes
> are a means for programs that only ever do IO in large(ish) blocks to
> take advantage of a hardware feature that also wants those large
> blocks.

I (vaguely) agree ith that.

> And only if the file mapping is in the correct state, and the
> program is willing to *maintain* them in the correct state to get the
> better performance.

I kinda agree with that, but the maintain is a bit hard as general
rule of thumb as file mappings can change behind the applications
back.  So building interfaces around the concept that there are
entirely stable mappings seems like a bad idea.

> I don't want xfs to grow code to write zeroes to
> mapped blocks just so it can then write-untorn to the same blocks.

Agreed.