block: document usage of bio iterator helpers
Now multi-page bvec is supported, some helpers may return page by page, meantime some may return segment by segment, this patch documents the usage. Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Omar Sandoval <osandov@fb.com> Signed-off-by: Ming Lei <ming.lei@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
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@ -117,3 +117,28 @@ Other implications:
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size limitations and the limitations of the underlying devices. Thus
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there's no need to define ->merge_bvec_fn() callbacks for individual block
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drivers.
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Usage of helpers:
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=================
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* The following helpers whose names have the suffix of "_all" can only be used
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on non-BIO_CLONED bio. They are usually used by filesystem code. Drivers
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shouldn't use them because the bio may have been split before it reached the
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driver.
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bio_for_each_segment_all()
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bio_first_bvec_all()
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bio_first_page_all()
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bio_last_bvec_all()
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* The following helpers iterate over single-page segment. The passed 'struct
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bio_vec' will contain a single-page IO vector during the iteration
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bio_for_each_segment()
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bio_for_each_segment_all()
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* The following helpers iterate over multi-page bvec. The passed 'struct
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bio_vec' will contain a multi-page IO vector during the iteration
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bio_for_each_bvec()
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rq_for_each_bvec()
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