block: cleanup and fix batch completion adding conditions

The conditions for whether or not a request is allowed adding to a
completion batch are a bit hard to read, and they also have a few
issues. One is that ioerror may indeed be a random value on passthrough,
and it's being checked unconditionally of whether or not the given
request is a passthrough request or not.

Rewrite the conditions to be separate for easier reading, and only check
ioerror for non-passthrough requests. This fixes an issue with bio
unmapping on passthrough, where it fails getting added to a batch. This
both leads to suboptimal performance, and may trigger a potential
schedule-under-atomic condition for polled passthrough IO.

Fixes: f794f33 ("block: add support for blk_mq_end_request_batch()")
Link: https://lore.kernel.org/r/20575f0a-656e-4bb3-9d82-dec6c7e3a35c@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

include/linux/blk-mq.h

@@ -861,12 +861,22 @@ static inline bool blk_mq_add_to_batch(struct request *req,
 				       void (*complete)(struct io_comp_batch *))
 {
 	/*
-	 * blk_mq_end_request_batch() can't end request allocated from
-	 * sched tags
+	 * Check various conditions that exclude batch processing:
+	 * 1) No batch container
+	 * 2) Has scheduler data attached
+	 * 3) Not a passthrough request and end_io set
+	 * 4) Not a passthrough request and an ioerror
 	 */
-	if (!iob || (req->rq_flags & RQF_SCHED_TAGS) || ioerror ||
-			(req->end_io && !blk_rq_is_passthrough(req)))
+	if (!iob)
 		return false;
+	if (req->rq_flags & RQF_SCHED_TAGS)
+		return false;
+	if (!blk_rq_is_passthrough(req)) {
+		if (req->end_io)
+			return false;
+		if (ioerror < 0)
+			return false;
+	}
 
 	if (!iob->complete)
 		iob->complete = complete;