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From:   Andreas Gruenbacher <agruenba@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>
Cc:     Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>,
        cluster-devel@redhat.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, ocfs2-devel@oss.oracle.com,
        Andreas Gruenbacher <agruenba@redhat.com>
Subject: [PATCH v6 14/19] iomap: Support partial direct I/O on user copy failures
Date:   Thu, 19 Aug 2021 21:40:57 +0200
Message-Id: <20210819194102.1491495-15-agruenba@redhat.com>
In-Reply-To: <20210819194102.1491495-1-agruenba@redhat.com>
References: <20210819194102.1491495-1-agruenba@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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@redhat.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;
+			}
+
 			/* 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