From: Jens Axboe <jens.axboe@xxxxxxxxxx> Subject: Re: [PATCH] blk: missing add of padded bytes to io completion byte count Date: Thu, 6 Mar 2008 14:41:39 +0100 > On Thu, Mar 06 2008, FUJITA Tomonori wrote: > > On Wed, 05 Mar 2008 09:21:24 -0600 > > James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > On Wed, 2008-03-05 at 14:51 +0100, Jens Axboe wrote: > > > > On Wed, Mar 05 2008, Tejun Heo wrote: > > > > > This is getting insanely subtle. Let's say there's PIO driver which > > > > > transfer certain sized chunks at a time and completes request partially > > > > > after completing each chunk and the driver uses draining to eat up > > > > > whatever excess data, which seems like a legit use case to me. But it > > > > > won't work because __end_that_request_first() will terminate when it > > > > > reaches reaches the 'true' transfer size. That's just broken API. FWIW, > > > > > > > > > > Nacked-by: Tejun Heo <htejun@xxxxxxxxx> > > > > > > > > Yeah, I think I may have gone a bit overboard in applying this so > > > > quickly. It's just not a good interface, silently adding the extra > > > > length if asked to complete more. It may even happen right now, for a > > > > driver that does no padding (it probably wont do any harm here either, > > > > but still). > > > > > > > > I'll try and see if I can come up with something cleaner. > > > > > > > > My basic design paradigm for this is that the _driver_ (or mid layer, if > > > > SCSI wants to handle it) should care about the padding. So make it easy > > > > for them to pad, but have it 'unrolled' by completion time. We should > > > > NOT need any extra_len checks or additions in the block/ directory, > > > > period. > > > > > > Right, that's why my original proposal was to do nothing for padding > > > (other than ensure the driver could adjust the length if it wanted to) > > > and to add an extra element always for draining, which the driver could > > > ignore. It basically pushed the use paradigm onto the driver. > > > > > > 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? What's the situation with this fix? -- 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
