[PATCH] block: improve error checking in blkdev_bio_end_io_async()

[Jens Axboe <axboe@xxxxxxxxx>]

Track the current error status of the dio with DIO_ERROR in the flags,
which can then avoid diving into dio->bio for the fast path of not
having any errors. This reduces the overhead of the function nicely,
which was previously dominated by this seemingly cheap check:

     4.55%     -1.13%  [kernel.vmlinux]  [k] blkdev_bio_end_io_async

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

---

diff --git a/block/fops.c b/block/fops.c
index d4f4fffb7d32..21d265caecff 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -127,6 +127,7 @@ enum {
 	DIO_MULTI_BIO		= 1,
 	DIO_SHOULD_DIRTY	= 2,
 	DIO_IS_SYNC		= 4,
+	DIO_ERROR		= 8,
 };
 
 struct blkdev_dio {
@@ -147,8 +148,10 @@ static void blkdev_bio_end_io(struct bio *bio)
 	struct blkdev_dio *dio = bio->bi_private;
 	unsigned int flags = dio->flags;
 
-	if (bio->bi_status && !dio->bio.bi_status)
+	if (!(flags & DIO_ERROR) && !dio->bio.bi_status) {
 		dio->bio.bi_status = bio->bi_status;
+		flags |= DIO_ERROR;
+	}
 
 	if (!(flags & DIO_MULTI_BIO) || atomic_dec_and_test(&dio->ref)) {
 		if (!(flags & DIO_IS_SYNC)) {
@@ -157,7 +160,7 @@ static void blkdev_bio_end_io(struct bio *bio)
 
 			WRITE_ONCE(iocb->private, NULL);
 
-			if (likely(!dio->bio.bi_status)) {
+			if (likely(!(flags & DIO_ERROR))) {
 				ret = dio->size;
 				iocb->ki_pos += ret;
 			} else {

-- 
Jens Axboe