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

Tejun Heo
2009-05-12T15:21:25Z

Hello, James.

James Bottomley wrote:
>> I think it's more like how many bytes are valid where the validity is
>> defined as the number of meaningful bytes on dev -> host commands and
>> the number of bytes the device actually consumed on the other
>> direction. Please note that this is different from the number of
>> bytes transferred due to padding or under other error conditions.
>
> For failed commands we don't have that information. All we know is how
> many bytes were actually transferred (because the HBA keeps a count), so
> it's the actual transfer count we use to construct the residual. No
> imputation of validity or otherwise. It just says I transferred this
> amount, based on the error make of it what you will.

Hmmm... so two separate meaning depending on command success/failure?
I doubt high level code or user application can make much use out of
raw transfer byte count. It doesn't mean all that much as it varies
depending on specific transport protocol in use.

>>>> * What about corner values? What does 0 or full resid count on
>>>> failure mean?
>>> 0 means everything transferred, full residual means nothing did.
>> Yeap, I was wondering about the combination 0 resid count + failure.
>> What would it mean? All bytes are valid but the command failed?
>
> Well, there are certain SCSI conditions called deferred errors and
> the like where we return Check Condition but everything's OK,
> redisual count should be zero, same goes for recovered errors
> ... there are actually lots of things we can get back as an "error"
> which means I'm warning you of something, but the transfer was OK.

Shouldn't those be request successful w/ sense data? Please note that
the term "error" in this context means failure of block layer request
not SCSI layer CHECK SENSE.

> Likewise we get unit attentions (essentialy AENs) which mean I'm
> telling you something before you start, so please try again. Here
> residual would be the full transfer.

And these are request failures.

> Also, we have the nasty USB case where no error return but an actual
> residual tells you something really went wrong.

This is something to be interpreted by lld, be it usb or scsi layer,
from block layer POV, if it's partial completion, it's success w/
residue count. If nothing really happened, it's request failure.

>> The behavior wasn't guaranteed before the change in paths including
>> SG_IO fast fail one. libata and ide have been and are completely
>> funky about residual counts anyway so I highly doubt anyone has been
>> depending on it.
>>
>> There's nothing wrong with keeping the original behavior in itself but
>> to me it looks like it would be a bad precedence when no one should
>> depend on the behavior.
>
> OK, that's what we'll do then, thanks.

I'm still reluctant to do it because...

* Its definition still isn't clear (well, at least to me) and if it's
defined as the number of valid bytes on request success and the
number of bytes HBA transferred on request failure, I don't think
it's all that useful.

* Seen from userland, residue count on request failure has never been
guaranteed and there doesn't seem to be any valid in kernel user.

* It would be extra code explicitly setting the residue count to full
on failure path. If it's something necessary, full residue count on
failure needs to be made default. If not, it will only add more
confusion.

Thanks.

--
tejun