Re: [PATCH 03/11] block: add rq->resid_len

James Bottomley
2009-05-12T03:43:24Z

On Tue, 2009-05-12 at 09:19 +0900, Tejun Heo wrote:
> Hello, all.
>
> James Bottomley wrote:
> >>> Does resid_len make any sense w/ failed requests? I think we would be
> >>> better off with declaring residual count to be undefined on request
> >>> failure. Is there any place which depends on it?
> >> IIRC, I wrote the code. I think that this doesn't matter but it's
> >> better not to change the behavior unless Eric ack on this change
> >> (maybe LSI has some management binary that assume this behavior though
> >> it's unlikely).
> >
> > Actually, yes it does, for many possible reasons.
> >
> > The first being if the device is too stupid to report an actual sector
> > location the next best way of determining where the error occurred is
> > from the residual. We don't make use of this in kernel (perhaps we
> > should?) but some of the user space programs for CD/DVD burning do.
>
> Really? Residual count on command success is used but on failure?
> That's a dangerous territory. When a SG_IO fails, the only data the
> app should be accessing is the sense data if the status indicates its
> validity. The problems with residual count on failed command are...
>
> * Not well defined. What does it mean really? It can't indicate
> successful partial transfer. If the request partially succeeded,
> the required behavior is to successfully complete the request
> partially with residual count and then fail the latter part when
> issued again. If the failure applies to the whole request but
> location information is useful, it should be carried in the sense
> data.

The definition is the amount of data transfer requested less the actual
that went over the wire ... that's certainly a well defined quantity;
although, one could argue about what this means in the device.
Certainly I agree that just because the data was transferred to or from
the device is no guarantee that the device did anything with it (or
transferred it accurately).

> * What about corner values? What does 0 or full resid count on
> failure mean?

0 means everything transferred, full residual means nothing did.

> * Different layers of failing. In SG_IO interface, a request may fail
> with -EIO way before it reaches block layer. Residual count can't
> be set to any meaningful value in these cases. We can set it to
> full count for these fast fail paths, but do we really wanna go
> there? Another problem is when a driver is missing SG_IO
> capability. Who's responsible for setting resid count in that case?
> How is upper layer gonna determine a SG_IO failed because lower
> level driver didn't support it or it genuinely failed?

Well, I prefer the concept of transfer length, which would be
initialised to zero ... however, residuals should be initialised to the
actual transfer count.

> I think it's just silly to give any meaning to resid count when the
> request fails. It's best to leave the field unmodified or just
> declare it undefined.

It's current behaviour. Technically that makes it part of the SG_IO
ABI ... although it could be deprecated if someone can verify there are
no current users.

James