Hello, James Bottomley wrote: >> No, it doesn't. Drain needs to extend the sg table too > > The patches do extend the sg table. Hmm... Where? >> and it makes controllers lax about buffer overruns for commands they shouldn't be. > > So adjust the qc->nbytes to show no drain element in libata ... although > the extra element gets mapped, the HBA will be none the wiser if there's > zero length allocated to it. Thus the entire system behaves exactly > like the drain element isn't present. The block layer is really just > making the drain element available ... you don't have to use it. Adjusting qc->nbytes isn't enough. libata will have to trim at the end of sglist. With both draining and padding, it means libata will have to trim one sg entry and a few more bytes. Block layer is behaving differently and in a very noticeable way, it's sending inconsistent values in data_len and sg list. >> Gee.. What's the deal with extra four bytes? If you're worried about >> the size of struct request, chopping off PC request part into a separate >> struct would be far better strategy than making what's already confusing >> more confusing. > > Well, we are trying to slim down the request structure and scsi_cmnd > structure. However, there's no inherent advantage to counting this all > in the block layer. Think of it just as a transparent passthrough. > You've told the block layer to make sure you have room for a drain > element when it constructs the SG list, which it does. Whether you use > it (by adding blk_rq_drain_size() on to the actual length you hand down) > or not is entirely up to you ... there's no need to push the decision on > that one into the block layer, since use or not is pretty much a cost > free thing. Okay, I see your point. The biggest problem is that by doing that blk layer breaks a pretty important rule - keeping data len consistent with sg list and the bugs it's gonna cause will be very subtle and dangerous. This is definitely where we can spend four extra bytes. 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
