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

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Message-ID: <20241213144740.GA17593@lst.de>
Date: Fri, 13 Dec 2024 15:47:40 +0100
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: 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 v2 2/7] iomap: Add zero unwritten mappings dio support

On Thu, Dec 12, 2024 at 12:40:07PM -0800, Darrick J. Wong wrote:
> > However, I still think that we should be able to atomic write mixed extents,
> > even though it is a pain to implement. To that end, I could be convinced
> > again that we don't require it...
> 
> Well... if you /did/ add a few entries to include/uapi/linux/fs.h for
> ways that an untorn write can fail, then we could define the programming
> interface as so:
> 
> "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.

> ...since there have been a few people who have asked about that ability
> so that they can write+fdatasync without so much overhead from file
> metadata updates.

And all of them fundamentally misunderstood file system semantics and/or
used weird bypasses that are dommed to corrupt the file system sooner
or later.