Jan's description for this patch is much better than mine, so I'm
quoting it verbatim here:

DAX currently doesn't set errors in the mapping when cache flushing
fails in dax_writeback_mapping_range(). Since this function can get
called only from fsync(2) or sync(2), this is actually as good as it can
currently get since we correctly propagate the error up from
dax_writeback_mapping_range() to filemap_fdatawrite(). However in the
future better writeback error handling will enable us to properly report
these errors on fsync(2) even if there are multiple file descriptors
open against the file or if sync(2) gets called before fsync(2). So
convert DAX to using standard error reporting through the mapping.

Signed-off-by: Jeff Layton <redacted>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-and-Tested-by: Ross Zwisler [off-list ref]
---
 fs/dax.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index 85abd741253d..9b6b04030c3f 100644
--- a/fs/dax.c
+++ b/fs/dax.c

@@ -901,8 +901,10 @@ int dax_writeback_mapping_range(struct address_space *mapping,
 
 			ret = dax_writeback_one(bdev, mapping, indices[i],
 					pvec.pages[i]);
-			if (ret < 0)
+			if (ret < 0) {
+				mapping_set_error(mapping, ret);
 				return ret;
+			}
 		}
 	}
 	return 0;

-- 
2.9.3