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

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Message-ID: <20251106135212.GA10477@lst.de>
Date: Thu, 6 Nov 2025 14:52:12 +0100
From: Christoph Hellwig <hch@....de>
To: Florian Weimer <fweimer@...hat.com>
Cc: Hans Holmberg <hans.holmberg@....com>, linux-xfs@...r.kernel.org,
	Carlos Maiolino <cem@...nel.org>,
	Dave Chinner <david@...morbit.com>,
	"Darrick J . Wong" <djwong@...nel.org>,
	Christoph Hellwig <hch@....de>, 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 Thu, Nov 06, 2025 at 02:48:12PM +0100, Florian Weimer wrote:
> * Hans Holmberg:
> 
> > We don't support preallocations for CoW inodes and we currently fail
> > with -EOPNOTSUPP, but this causes an issue for users of glibc's
> > posix_fallocate[1]. If fallocate fails, posix_fallocate falls back on
> > writing actual data into the range to try to allocate blocks that way.
> > That does not actually gurantee anything for CoW inodes however as we
> > write out of place.
> 
> Why doesn't fallocate trigger the copy instead?  Isn't this what the
> user is requesting?

What copy?