Re: [PATCH 4/4] iomap: Use FUA for pure data O_DSYNC DIO writes

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On Wed 18-04-18 14:08:28, Dave Chinner wrote:
> @@ -1012,8 +1035,16 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  			dio->flags |= IOMAP_DIO_DIRTY;
>  	} else {
>  		dio->flags |= IOMAP_DIO_WRITE;
> -		if (iocb->ki_flags & IOCB_DSYNC)
> +		if (iocb->ki_flags & IOCB_DSYNC) {
>  			dio->flags |= IOMAP_DIO_NEED_SYNC;
> +			/*
> +			 * We optimistically try using FUA for this IO.  Any
> +			 * non-FUA write that occurs will clear this flag, hence
> +			 * we know before completion whether a cache flush is
> +			 * necessary.
> +			 */
> +			dio->flags |= IOMAP_DIO_WRITE_FUA;
> +		}

So I don't think this is quite correct. IOCB_DSYNC gets set also for O_SYNC
writes (in that case we also set IOCB_SYNC). And for those we cannot use
the FUA optimization AFAICT (definitely IOMAP_F_DIRTY isn't a safe
indicator of a need of full fsync for O_SYNC). Other than that the patch
looks good to me.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR



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