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

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Message-ID: <20251106170501.GA25601@lst.de>
Date: Thu, 6 Nov 2025 18:05:01 +0100
From: Christoph Hellwig <hch@....de>
To: Florian Weimer <fweimer@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>, Christoph Hellwig <hch@....de>,
	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>,
	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 05:31:28PM +0100, Florian Weimer wrote:
> It's been a few years, I think, and maybe we should drop the allocation
> logic from posix_fallocate in glibc?  Assuming that it's implemented
> everywhere it makes sense?

I really think it should go away.  If it turns out we find cases where
it was useful we can try to implement a zeroing fallocate in the kernel
for the file system where people want it.  gfs2 for example currently
has such an implementation, and we could have somewhat generic library
version of it.

> There are more always-CoW, compressing file
> systems these days, so applications just have to come to terms with the
> fact that even after posix_fallocate, writes can still fail, and not
> just because of media errors.

Yes.