iomap: support ioends for direct reads

Support using the ioend structure to defer I/O completion for direct
reads in addition to writes.  This requires a check for the operation
to not merge reads and writes in iomap_ioend_can_merge.  This support
will be used for bounce buffered direct I/O reads that need to copy
data back to the user address space on read completion.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

fs/iomap/ioend.c

@@ -299,6 +299,14 @@ EXPORT_SYMBOL_GPL(iomap_finish_ioends);
 static bool iomap_ioend_can_merge(struct iomap_ioend *ioend,
 		struct iomap_ioend *next)
 {
+	/*
+	 * There is no point in merging reads as there is no completion
+	 * processing that can be easily batched up for them.
+	 */
+	if (bio_op(&ioend->io_bio) == REQ_OP_READ ||
+	    bio_op(&next->io_bio) == REQ_OP_READ)
+		return false;
+
 	if (ioend->io_bio.bi_status != next->io_bio.bi_status)
 		return false;
 	if (next->io_flags & IOMAP_IOEND_BOUNDARY)