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

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Date:   Thu, 9 Sep 2021 12:20:46 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>, cluster-devel@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v7 15/19] iomap: Support partial direct I/O on user copy
 failures

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@...hat.com>
> ---
>  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;

Do we need a NOWAIT check here to skip the wait_for_completion
for that case?