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Date: Mon, 10 Nov 2025 10:38:28 +0100
From: Christoph Hellwig <hch@lst.de>
To: Florian Weimer <fweimer@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>, Christoph Hellwig <hch@lst.de>,
	Matthew Wilcox <willy@infradead.org>,
	Hans Holmberg <hans.holmberg@wdc.com>, linux-xfs@vger.kernel.org,
	Carlos Maiolino <cem@kernel.org>,
	"Darrick J . Wong" <djwong@kernel.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	libc-alpha@sourceware.org
Subject: Re: [RFC] xfs: fake fallocate success for always CoW inodes
Message-ID: <20251110093828.GC22674@lst.de>
References: <20251106133530.12927-1-hans.holmberg@wdc.com> <lhuikfngtlv.fsf@oldenburg.str.redhat.com> <20251106135212.GA10477@lst.de> <aQyz1j7nqXPKTYPT@casper.infradead.org> <lhu4ir7gm1r.fsf@oldenburg.str.redhat.com> <20251106170501.GA25601@lst.de> <878qgg4sh1.fsf@mid.deneb.enyo.de> <aRESlvWf9VquNzx3@dread.disaster.area> <lhuseem1mpe.fsf@oldenburg.str.redhat.com>
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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.