rbd: simplify rbd_rq_fn()
When processing a request, rbd_rq_fn() makes clones of the bio's in the request's bio chain and submits the results to osd's to be satisfied. If a request bio straddles the boundary between objects backing the rbd image, it must be represented by two cloned bio's, one for the first part (at the end of one object) and one for the second (at the beginning of the next object). This has been handled by a function bio_chain_clone(), which includes an interface only a mother could love, and which has been found to have other problems. This patch defines two new fairly generic bio functions (one which replaces bio_chain_clone()) to help out the situation, and then revises rbd_rq_fn() to make use of them. First, bio_clone_range() clones a portion of a single bio, starting at a given offset within the bio and including only as many bytes as requested. As a convenience, a request to clone the entire bio is passed directly to bio_clone(). Second, bio_chain_clone_range() performs a similar function, producing a chain of cloned bio's covering a sub-range of the source chain. No bio_pair structures are used, and if successful the result will represent exactly the specified range. Using bio_chain_clone_range() makes bio_rq_fn() a little easier to understand, because it avoids the need to pass very much state information between consecutive calls. By avoiding the need to track a bio_pair structure, it also eliminates the problem described here: http://tracker.newdream.net/issues/2933 Note that a block request (and therefore the complete length of a bio chain processed in rbd_rq_fn()) is an unsigned int, while the result of rbd_segment_length() is u64. This change makes this range trunctation explicit, and trips a bug if the the segment boundary is too far off. Signed-off-by: NAlex Elder <elder@inktank.com> Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>
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