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: <20251030055851.GA12703@lst.de>
Date: Thu, 30 Oct 2025 06:58:51 +0100
From: Christoph Hellwig <hch@....de>
To: Qu Wenruo <wqu@...e.com>
Cc: Christoph Hellwig <hch@....de>, "Darrick J. Wong" <djwong@...nel.org>,
	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, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 4/4] xfs: fallback to buffered I/O for direct I/O when
 stable writes are required

On Thu, Oct 30, 2025 at 07:53:30AM +1030, Qu Wenruo wrote:
> Yep, a common helper will help, or even integrate the check into 
> __iomap_dio_rw().

Having the check in __iomap_dio_rw would be a last resort, because at
the point we've already done direct I/O specific locking we'd need to
unwind from, making the fallback slower than we'd have to.

> However I'm not sure if a warning will be that useful.
>
> If the warning is only outputted once like here, it doesn't show the ino 
> number to tell which file is affected.
> If the warning is shown every time, it will flood the dmesg.

While the flag is set on the address_space it is global (or semi global
for separate storage pools like the XFS RT device), so the inode number
doesn't really matter too much.

> It will be much straightforward if there is some flag allowing us to return 
> error directly if true zero-copy direct IO can not be executed.

I don't really understand this part.