linux-kernel - Re: [RFC] xfs: fake fallocate success for always CoW inodes

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Message-ID: <20251110093828.GC22674@lst.de>
Date: Mon, 10 Nov 2025 10:38:28 +0100
From: Christoph Hellwig <hch@....de>
To: Florian Weimer <fweimer@...hat.com>
Cc: Dave Chinner <david@...morbit.com>, Christoph Hellwig <hch@....de>,
	Matthew Wilcox <willy@...radead.org>,
	Hans Holmberg <hans.holmberg@....com>, linux-xfs@...r.kernel.org,
	Carlos Maiolino <cem@...nel.org>,
	"Darrick J . Wong" <djwong@...nel.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	libc-alpha@...rceware.org
Subject: Re: [RFC] xfs: fake fallocate success for always CoW inodes

On Mon, Nov 10, 2025 at 06:27:41AM +0100, Florian Weimer wrote:
> Sorry, I made the example confusing.
> 
> How would the application deal with failure due to lack of fallocate
> support?  It would have to do a pwrite, like posix_fallocate does to
> today, or maybe ftruncate.  This is way I think removing the fallback
> from posix_fallocate completely is mostly pointless.

In general it would ftruncate.  If it thinks it can't work without
preallocation at all the application will fail, as again the lack
of posix_fallocate means that space can't be preallocated.