From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: [PATCH RFC] yet more struct scsi_lun Date: Fri, 4 Nov 2005 08:37:19 +0100 Message-ID: <20051104073718.GY26049@suse.de> References: <20051023043301.GA22615@havoc.gtf.org> <20051023070011.GA26569@havoc.gtf.org> <435B6A62.8070306@torque.net> <435BBD9A.80603@pobox.com> <20051024075934.GK2811@suse.de> <43625EC3.9060708@cs.wisc.edu> <20051031102451.GR19267@suse.de> <436AC620.20505@cs.wisc.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from ns.virtualhost.dk ([195.184.98.160]:58384 "EHLO virtualhost.dk") by vger.kernel.org with ESMTP id S1030475AbVKDHgd (ORCPT ); Fri, 4 Nov 2005 02:36:33 -0500 Content-Disposition: inline In-Reply-To: <436AC620.20505@cs.wisc.edu> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Mike Christie Cc: Jeff Garzik , dougg@torque.net, linux-scsi@vger.kernel.org On Thu, Nov 03 2005, Mike Christie wrote: > Jens Axboe wrote: > > > >On the SCSI side, I would suggest just making shost->max_sectors set > >q->max_hw_sectors and leave q->max_sectors to some generic kernel-wide > >block layer define (of course making sure that ->max_sectors <= > > If the value for this block layer define was around 16,000 sectors, > would that be ok? The reason I ask is becuase when I get ..... Nope :) > >->max_hw_sectors). That's the easy part. > > > >The bio_add_page() stuff is a little trickier, since it wants to know if > >this is fs or 'generic' io. For fs io, we would like to cap the building > >of the bio to ->max_sectors, but for eg SG_IO issued io it should go as > >high as ->max_hw_sectors. Perhaps the easiest is just to have > >bio_fs_add_page() and bio_pc_add_page(), each just passing in the max > >value as an integer to bio_add_page(). But it's not exactly pretty. > > > >The ll_rw_blk.c merging is easy, since you don't need to do anything > >there. It should test against ->max_sectors as it already does, since > >this (sadly) is still the primary way we build large ios. > > > > .... here, I am running into a problem. Basically, as you know the > largest BIO we can make is 1 MB due to BIO_MAX_PAGES, and for st and sg > we need to support commands around 6 MB, so we would have a request with > 6 BIOs. To make this monster request I wanted to use the block layer > functions and do something like this: > > + for (i = 0; i < nsegs; i++) { > + bio = bio_map_pages(q, sg[i].page, sg[i].length, > sg[i].offset, gfp); > + if (IS_ERR(bio)) { > + err = PTR_ERR(bio); > + goto free_bios; > + } > + len += sg[i].length; > + > + bio->bi_flags &= ~(1 << BIO_SEG_VALID); > + if (rq_data_dir(rq) == WRITE) > + bio->bi_rw |= (1 << BIO_RW); > + blk_queue_bounce(q, &bio); > + > + if (i == 0) > + blk_rq_bio_prep(q, rq, bio); > > /* hope to carve out the __make_request code that does the below > operations and make a fucntion that can be shared */ > > + else if (!q->back_merge_fn(q, rq, bio)) { > + err = -EINVAL; > + bio_endio(bio, bio->bi_size, 0); > + goto free_bios; > + } else { > + rq->biotail->bi_next = bio; > + rq->biotail = bio; > + rq->hard_nr_sectors += bio_sectors(bio); > + rq->nr_sectors = rq->hard_nr_sectors; > > ........ > > But since q->back_merge_fn() tests against q->max_sectors, it must be a > high value so that we can merge in those BIOs. I mean if q->max_sectors > is some reasonable number like only 1024 sectors, q->back_merge_fn will > return a failure. Should I instead seperate ll_back_merge_fn into two > functions, one that checks the sectors and one that checks the segments > or if ll_back_merge_fn tested for max_hw_sectors we would be ok too? It will take a whole lot of factoring out to make this happen I fear, the results will not be easier for the eyes. How about just keeping it simple - add a bio and request flag that basically just says "don't honor soft max size" and whenever that is set, you check for ->max_hw_sectors instead of ->max_sectors? Might even be enough to just do this for the request and just require stuffing more bio's into the request. But the bio has plenty of flag room left, so... -- Jens Axboe