Re: [PATCH v2 2/2] block: only use REQ_FUA for direct writes if the device supports it

[Christoph Hellwig <hch@infradead.org>]

On Sat, Aug 15, 2026 at 08:02:15PM +0800, Zhenxian Ma wrote:
> From: Zhenxian Ma <mzx199711@gmail.com>
> 
> When a block device does not support FUA natively, the block layer
> emulates it by adding a cache flush to every write bio.  An O_DSYNC
> direct write that spans N bios then costs N flushes, rather than the
> single generic_write_sync() issued after the write completes.
> 
> Introduce blkdev_dio_fua() to decide when REQ_FUA is set.  A synchronous
> write can rely on generic_write_sync() when the device lacks FUA, so it
> sets REQ_FUA only when bdev_fua() is true.  An asynchronous write
> completes in blkdev_bio_end_io() and cannot call the blocking
> generic_write_sync(), so it keeps REQ_FUA (emulated when needed) to stay
> durable.
> 
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Zhenxian Ma <mzx199711@gmail.com>
> Signed-off-by: Zhenxian Ma <mazhenxian@xiaohongshu.com>
> ---
>  block/fops.c | 32 +++++++++++++++++++++++++++-----
>  1 file changed, 27 insertions(+), 5 deletions(-)
> 
> diff --git a/block/fops.c b/block/fops.c
> index a2c3af38106b..e7d073620271 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -26,12 +26,26 @@ static inline struct inode *bdev_file_inode(struct file *file)
>  	return file->f_mapping->host;
>  }
>  
> -static blk_opf_t dio_bio_write_op(struct kiocb *iocb)
> +static bool blkdev_dio_fua(struct kiocb *iocb, struct block_device *bdev)
> +{
> +	if (!iocb_is_dsync(iocb))
> +		return false;
> +	/*
> +	 * Async writes cannot fall back to generic_write_sync(), so they must
> +	 * use FUA (emulated if needed); sync writes only need it when the
> +	 * device supports FUA natively.
> +	 */

Ah, I guess this explain why the original code unconditionally uses FUA.
Let's hope no one cares about the performance of large async writes
on devices without FUA.

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>