FUJITA Tomonori wrote: > On Fri, 07 Mar 2008 09:07:23 +0900 > Tejun Heo <htejun@xxxxxxxxx> wrote: > >> Jens Axboe wrote: >>>>> If we want the use paradigm shared between block and driver, then I >>>>> think the best approach is to keep all the bios the same (so not adjust >>>>> for padding), but do adjust in the blk_rq_map_sg(). That way we have >>>>> the padding and draining unwind information by comparing with the bio. >>>> Adjusting only sg in blk_rq_map_sg (like drain) looks much >>>> better. This works with libata for me. >>> Looks like a much better solution to me. Anyone have any valid >>> objections against moving the padding to the sg map time? >> Not necessarily objections but some concerns. >> >> * As completion is done in bio terms, it makes completion from LLDs a >> bit cumbersome, but this is unavoidable if we break sum(bio) == sum(sg). > > What do you mean? How does sub(bio) affect LLDs? LLDs which loop over sg's trying to complete rq incrementally will see rq going away sooner than it expected. >> * I've been wondering why we are not using sg chain / table or whatever >> directly in bios and maybe rq_map_sg can go away in future. > > You mean that LLDs use bios directly? For me, sg and bio have very > different objectives and it's a clean layer separation. Actually the other way, block layer use sg instead of bio_vec in bio. Layer separation doesn't necessarily require copying about the same information to differently formatted data structure. I'm not sure it will be a clean win tho. Requests hang longer in scheduler queue and and bio_vec is smaller and scatterlist. The thing is that, to me, blk_rq_map_sg() doesn't really look necessary, it can be done just as well when the request is fetched from the queue by block driver. (continued below...) >> How about separating out the padding / draining adjustment into a >> separate interface? Say, blk_rq_apply_extra() and blk_rq_undo_extra() >> and make it the responsibility of the LLD which requested >> padding/draining to apply and undo the adjustments? It can undo the >> adjustments when it returns the the request to its upper layer. If rq >> completion is handled by upper layer, it will do the right thing. If rq >> completion is handled by LLD, it can see the bio it wants to see. > > If possible, I'd like to avoid creating APIs for them. I think that > the current approach is much better than such APIs. And, so, I'm not too sure whether putting more mechanisms into it is a good idea. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
