block: avoid extra bio reference for async O_DIRECT
The bio referencing has a trick that doesn't do any actual atomic
inc/dec on the reference count until we have to elevator to > 1. For the
async IO O_DIRECT case, we can't use the simple DIO variants, so we use
__blkdev_direct_IO(). It always grabs an extra reference to the bio
after allocation, which means we then enter the slower path of actually
having to do atomic_inc/dec on the count.
We don't need to do that for the async case, unless we end up going
multi-bio, in which case we're already doing huge amounts of IO. For the
smaller IO case (< BIO_MAX_PAGES), we can do without the extra ref.
Based on an earlier patch (and commit log) from Jens Axboe.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fs/block_dev.c b/fs/block_dev.c
index d233a59..e1886cc 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -302,7 +302,8 @@ static void blkdev_bio_end_io(struct bio *bio)
}
dio->iocb->ki_complete(iocb, ret, 0);
- bio_put(&dio->bio);
+ if (dio->multi_bio)
+ bio_put(&dio->bio);
} else {
struct task_struct *waiter = dio->waiter;
@@ -343,14 +344,15 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
return -EINVAL;
bio = bio_alloc_bioset(GFP_KERNEL, nr_pages, &blkdev_dio_pool);
- bio_get(bio); /* extra ref for the completion handler */
dio = container_of(bio, struct blkdev_dio, bio);
dio->is_sync = is_sync = is_sync_kiocb(iocb);
- if (dio->is_sync)
+ if (dio->is_sync) {
dio->waiter = current;
- else
+ bio_get(bio);
+ } else {
dio->iocb = iocb;
+ }
dio->size = 0;
dio->multi_bio = false;
@@ -400,6 +402,13 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
}
if (!dio->multi_bio) {
+ /*
+ * AIO needs an extra reference to ensure the dio
+ * structure which is embedded into the first bio
+ * stays around.
+ */
+ if (!is_sync)
+ bio_get(bio);
dio->multi_bio = true;
atomic_set(&dio->ref, 2);
} else {