linux-kernel - Re: [PATCH 3/4] xfs: use IOCB_DONTCACHE when falling back to buffered writes

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Message-ID: <20251029155734.GD3356773@frogsfrogsfrogs>
Date: Wed, 29 Oct 2025 08:57:34 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: 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 3/4] xfs: use IOCB_DONTCACHE when falling back to
 buffered writes

On Wed, Oct 29, 2025 at 08:15:04AM +0100, Christoph Hellwig wrote:
> Doing sub-block direct writes to COW inodes is not supported by XFS,
> because new blocks need to be allocated as a whole.  Such writes
> fall back to buffered I/O, and really should be using the
> IOCB_DONTCACHE that didn't exist when the code was added to mimic
> direct I/O semantics as closely as possible.  Also clear the
> IOCB_DIRECT flags so that later code can't get confused by it being
> set for something that at this point is not a direct I/O operation
> any more.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Seems like a reasonable fallback now that we have dontcache
Reviewed-by: "Darrick J. Wong" <djwong@...nel.org>

--D

> ---
>  fs/xfs/xfs_file.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 5703b6681b1d..e09ae86e118e 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1119,6 +1119,9 @@ xfs_file_write_iter(
>  		ret = xfs_file_dio_write(iocb, from);
>  		if (ret != -ENOTBLK)
>  			return ret;
> +
> +		iocb->ki_flags &= ~IOCB_DIRECT;
> +		iocb->ki_flags |= IOCB_DONTCACHE;
>  	}
>  
>  	if (xfs_is_zoned_inode(ip))
> -- 
> 2.47.3
> 
>