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

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Message-ID: <20251106124900.GA6144@lst.de>
Date: Thu, 6 Nov 2025 13:49:00 +0100
From: hch <hch@....de>
To: Johannes Thumshirn <Johannes.Thumshirn@....com>
Cc: "Darrick J. Wong" <djwong@...nel.org>, hch <hch@....de>,
	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 Thu, Nov 06, 2025 at 09:50:10AM +0000, Johannes Thumshirn wrote:
> 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.

qemu passes I/O through, so yes it is guest controller.  Windows is most
famous, but the Linux swap code can trigger it easily too.