On Tue, 20 Feb 2007, Mike Christie wrote: > I think you actually want max_hw_sectors. Well, you might and you might > not :) I think we do not. We don't care about the maximum transfer length the driver can theoretically support; we care about the maximum transfer length the system will allow. For example, if max_sectors is 240 and max_hw_sectors is 512 then the system will reject a attempted transfer of 300 sectors, even though the hardware is capable of carrying it out. > I think a estimate of the max transfer length would be more like: > > unsigned int max_segment_size, max_xfer, > int max_segments; > > /* > * does not account for any weird arch segments boundary limits > * or vmerge limits > */ > if (!(q->queue_flags & (1 << QUEUE_FLAG_CLUSTER))) > max_segment_size = PAGE_SIZE; > else > max_segment_size = q->max_segment_size; > > max_segments = min(q->max_hw_segments, q->max_phys_segments); > > max_xfer = min(q->max_hw_sectors * 512, > max_segments * max_segment_size); Isn't it possible that this multiplication might overflow? > > return max_xfer; > > > The problem is that we assume we will get nice large segments. When > using sg it will try to allocate multiple pages and make large segments. > We could hit a bad case where we cannot allocate enough large segments, > so a worst case would result in a max_segment_size of PAGE_SIZE: > > max_segments = min(q->max_hw_segments, q->max_phys_segments); > > max_xfer = min(q->max_hw_sectors * 512, > max_segments * PAGE_SIZE); > > return max_xfer; That is a very pessimistic estimate. I don't see any reason for using it, especially since sg sets aside a reserved buffer which guarantees that a certain minimum amount of memory will always be available. > I think you have to take into account the scatterlist limits because > some drivers like lpfc have a small sht->sg_tablesize/q->max_hw_segments > (64), but have a large q->max_hw_sectors (0xFFFF) and uses the default > q->max_sectors (1024). So in the worst case we could end up with a max > transfer size of only 64 * PAGE_SIZE and this a lot smaller than > q->max_sectors. Would you care to rewrite the patch? Keep in mind particularly that the SG_GET_RESERVED_SIZE ioctl is supposed to return the size of the reserved area, not the maximum transfer length. Alan Stern - 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
