On Thu, 2 Oct 2008 19:13:57 +0200 Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: > On Thu, Oct 02 2008, James Bottomley wrote: > > On Thu, 2008-10-02 at 18:58 +0200, Jens Axboe wrote: > > > On Thu, Oct 02 2008, James Bottomley wrote: > > > > The bug would appear to be that we sometimes only look at q->max_sectors > > > > when deciding on mergability. Either we have to insist on max_sectors > > > > <= hw_max_sectors, or we have to start using min(q->max_sectors, > > > > q->max_hw_sectors) for this. > > > > > > q->max_sectors MUST always be <= q->max_hw_sectors, otherwise we could > > > be sending down requests that are too large for the device to handle. So > > > that condition would be a big bug. The sysfs interface checks for this, > > > and blk_queue_max_sectors() makes sure that is true as well. > > > > Yes, that seems always to be enforced. Perhaps there are other ways of > > tripping this problem ... I'm still sure if it occurs it's because we do > > a physical merge where a virtual merge is forbidden. > > > > > The fixes proposed still look weird. There is no phys vs hw segment > > > constraints, the request must adhere to the limits set by both. It's > > > mostly a moot point anyway, as 2.6.28 will get rid of the hw accounting > > > anyway. > > > > Agree all three proposed fixes look wrong. However, if it's what I > > think, just getting rid of hw accounting won't fix the problem because > > we'll still have the case where a physical merge is forbidden by iommu > > constraints ... this still needs to be accounted for. > > > > What we really need is a demonstration of what actually is going > > wrong ... > > Yep, IIRC we both asked for that the last time as well... Nikanth? Possibly, blk_phys_contig_segment might miscalculate q->max_segment_size? blk_phys_contig_segment does: req->biotail->bi_size + next_req->bio->bi_size > q->max_segment_size; But it's possible that req->biotail and the previous bio are supposed be merged into one segment? Then we could create too large segment here. -- 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
