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

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Message-ID: <lhuikfngtlv.fsf@oldenburg.str.redhat.com>
Date: Thu, 06 Nov 2025 14:48:12 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Hans Holmberg <hans.holmberg@....com>
Cc: 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

* 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?

Thanks,
Florian