linux-kernel - Re: [PATCH 4/4] xfs: fallback to buffered I/O for direct I/O when stable writes are required

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Message-ID: <20251110135932.GA11277@lst.de>
Date: Mon, 10 Nov 2025 14:59:32 +0100
From: Christoph Hellwig <hch@....de>
To: "Nirjhar Roy (IBM)" <nirjhar.roy.lists@...il.com>
Cc: Christoph Hellwig <hch@....de>, Carlos Maiolino <cem@...nel.org>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-raid@...r.kernel.org,
	linux-block@...r.kernel.org
Subject: Re: [PATCH 4/4] xfs: fallback to buffered I/O for direct I/O when
 stable writes are required

On Mon, Nov 10, 2025 at 07:08:05PM +0530, Nirjhar Roy (IBM) wrote:
> Minor: Let us say that an user opens a file in O_DIRECT in an atomic
> write enabled device(requiring stable writes), we get this warning
> once. Now the same/different user/application opens another file
> with O_DIRECT in the same atomic write enabled device and expects
> atomic write to be enabled - but it will not be enabled (since the
> kernel has falled back to the uncached buffered write path)
> without any warning message. Won't that be a bit confusing for the
> user (of course unless the user is totally aware of the kernel's exact
> behavior)?

The kernel with this patch should reject IOCB_ATOMIC writes because
the FMODE_CAN_ATOMIC_WRITE is not set when we need to fallback.

But anyway, based on the feedback in this thread I plan to revisit the
approach so that the I/O issuer can declare I/O stable (initially just
for buffered I/O, but things like nvmet and nfsd might be able to
guarantee that for direct I/O as well), and then bounce buffer in lower
layers.  This should then also support parallel writes, async I/O and
atomic writes.