Re: [PATCH v7 15/19] iomap: Support partial direct I/O on user copy failures

[Darrick J. Wong]

On Fri, Aug 27, 2021 at 06:49:22PM +0200, Andreas Gruenbacher wrote:
> In iomap_dio_rw, when iomap_apply returns an -EFAULT error and the
> IOMAP_DIO_PARTIAL flag is set, complete the request synchronously and
> return a partial result.  This allows the caller to deal with the page
> fault and retry the remainder of the request.
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>

Pretty straightforward.
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>

--D

> ---
>  fs/iomap/direct-io.c  | 6 ++++++
>  include/linux/iomap.h | 7 +++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 8054f5d6c273..ba88fe51b77a 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -561,6 +561,12 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  		ret = iomap_apply(inode, pos, count, iomap_flags, ops, dio,
>  				iomap_dio_actor);
>  		if (ret <= 0) {
> +			if (ret == -EFAULT && dio->size &&
> +			    (dio_flags & IOMAP_DIO_PARTIAL)) {
> +				wait_for_completion = true;
> +				ret = 0;
> +			}
> +
>  			/* magic error code to fall back to buffered I/O */
>  			if (ret == -ENOTBLK) {
>  				wait_for_completion = true;
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 479c1da3e221..bcae4814b8e3 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -267,6 +267,13 @@ struct iomap_dio_ops {
>    */
>  #define IOMAP_DIO_OVERWRITE_ONLY	(1 << 1)
>  
> +/*
> + * When a page fault occurs, return a partial synchronous result and allow
> + * the caller to retry the rest of the operation after dealing with the page
> + * fault.
> + */
> +#define IOMAP_DIO_PARTIAL		(1 << 2)
> +
>  ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  		const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
>  		unsigned int dio_flags);
> -- 
> 2.26.3
>