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>
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@ -302,6 +302,7 @@ static void blkdev_bio_end_io(struct bio *bio)
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}
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dio->iocb->ki_complete(iocb, ret, 0);
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if (dio->multi_bio)
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bio_put(&dio->bio);
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} else {
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struct task_struct *waiter = dio->waiter;
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@ -343,14 +344,15 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
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return -EINVAL;
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bio = bio_alloc_bioset(GFP_KERNEL, nr_pages, &blkdev_dio_pool);
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bio_get(bio); /* extra ref for the completion handler */
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dio = container_of(bio, struct blkdev_dio, bio);
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dio->is_sync = is_sync = is_sync_kiocb(iocb);
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if (dio->is_sync)
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if (dio->is_sync) {
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dio->waiter = current;
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else
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bio_get(bio);
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} else {
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dio->iocb = iocb;
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}
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dio->size = 0;
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dio->multi_bio = false;
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@ -400,6 +402,13 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
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}
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if (!dio->multi_bio) {
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/*
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* AIO needs an extra reference to ensure the dio
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* structure which is embedded into the first bio
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* stays around.
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*/
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if (!is_sync)
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bio_get(bio);
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dio->multi_bio = true;
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atomic_set(&dio->ref, 2);
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} else {
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