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Date: Tue, 11 Nov 2025 09:56:59 +0100
From: Christoph Hellwig <hch@lst.de>
To: Dave Chinner <david@fromorbit.com>
Cc: Florian Weimer <fweimer@redhat.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: <20251111085659.GA11723@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> <aRJK5LqJnrT5KAyH@dread.disaster.area>
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On Tue, Nov 11, 2025 at 07:28:20AM +1100, Dave Chinner wrote:
> IOWs, I have no problems with COW filesystems not doing
> preallocation, but if they are going to return success they still
> need to perform all the non-allocation parts of fallocate()
> operations correctly.
> 
> Again, I don't see a need for a new API here to provide
> non-destructive "truncate up only" semantics as we already have
> those semantics built into the ALLOCATE_RANGE operation...

The problem it loses the ability of an intelligent application using
the low-level Linux API to probe what is there.  That might not be a
major issue, but it is an issue at least.