Re: [PATCH] block: fix residual byte count handling

[Tejun Heo <htejun@gmail.com>]

FUJITA Tomonori wrote:
>> Yeah, libata did its own padding and needed to add draining.  Private
>> implementation was complex as hell and James suggested moving them to
>> block layer.  Are you suggesting moving them back to drivers?
> 
> No, I'm not. I've been working on the IOMMUs to remove such
> workarounds in LLDs.
> 
> What drivers need to do on this is just adding a padding length, that
> is, drivers don't need to change the structure of the sg list (like
> splitting a sg entry), right? And it doesn't break the SAS drivers
> that support SATAPI, does it?
> 
> But I agree that drivers want to get a complete sglist so I'm fine
> with adjusting sglist entries in the block layer with your secode
> patch (separate out padding from alignment). As we discussed, I'm fine
> with breaking sum(sg) == rq->data_len as long as rq->data_len means
> the true data length.

As long as the second patch is in, what value rq->data_len indicates
doesn't matter to drivers which don't use explicit padding or draining,
so the situation is much more controlled.  I don't care which value
rq->data_len would indicate.  I'd prefer it equal sum(sg) as that value
is what IDE and libata which will be the major users of padding and/or
draining expect in rq->data_len but fixing up that shouldn't be too
difficult.  I guess this can be determined by Jens.  If Jens likes
rq->data_len to contain requested transfer size, I'll post updated patches.

>>>> buffer after it, it ends up with unaligned sg entry in the middle and
>>>> rq->data_len + rq->extra_len will overrun the sg entry after the drain
>>>> page which is really dangerous.
>>> The drivers know that they use drain buffer. They can take care about
>>> themselves on this too. If we want to do explicitly, we could have
>>> rq->pad_len and rq->drain_len instead of rq->extra_len, though I think
>>> that we are fine without these values because these drivers already
>>> tell the block layer what they want and know that the block layer
>>> gives it.
>> So, if a driver has requested aligning and draining, the driver should
>> extend the sg entry before the last one by the alignment if draining was
>> used for the request and extent the last sg if the draining wasn't used.
>>  I'd rather just implement them in the drivers.
> 
> The block layer extends the sg entry? The drivers just adjust
> sg->length?

Still, do you really wanna force such things into low level drivers?
That will be one extremely fragile API and will be really difficult to
tell when things go wrong.

-- 
tejun