linux-kernel - Re: fall back from direct to buffered I/O when stable writes are required

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Message-ID: <9530fca4-418d-4415-b365-cad04a06449b@wdc.com>
Date: Thu, 6 Nov 2025 09:50:10 +0000
From: Johannes Thumshirn <Johannes.Thumshirn@....com>
To: "Darrick J. Wong" <djwong@...nel.org>, hch <hch@....de>
CC: Jan Kara <jack@...e.cz>, Keith Busch <kbusch@...nel.org>, Dave Chinner
	<david@...morbit.com>, Carlos Maiolino <cem@...nel.org>, Christian Brauner
	<brauner@...nel.org>, "Martin K. Petersen" <martin.petersen@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: fall back from direct to buffered I/O when stable writes are
 required

On 11/5/25 10:44 PM, Darrick J. Wong wrote:
> Just out of curiosity -- is qemu itself mutating the buffers that it is
> passing down to the lower levels via dio?  Or is it a program in the
> guest that's mutating buffers that are submitted for dio, which then get
> zerocopied all the way down to the hypervisor?

If my memory serves me right it is the guest (or at least can be). I 
remember a bug report on btrfs where a Windows guest had messed up 
checksums because of modifying inflight I/O.