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Christoph Hellwig
531724abc3
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>
Linux kernel
============
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