Re: [PATCH 10/18] lightnvm: pblk: use bio_copy_kern when possible

[Javier GonzÃlez]

> On 7 Sep 2017, at 13.08, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> 
> On Wed, Sep 06, 2017 at 04:00:56PM +0200, Javier GonzÃlez wrote:
>>> Nope.  You want to loop over vmalloc_to_page and call bio_add_page
>>> for each page,
>> 
>> Yes. This is basically what I did before.
>> 
>>> after taking care of virtually tagged caches instead
>>> of this bounce buffering.
>> 
>> And thus I considered bio_copy_kern to be a better solution, since it
>> will through time take care of doing the vmalloc_to_page correctly for
>> all cases.
> 
> bio_copy_kern copies all the data, so it is generally not a good
> idea.  The cache flushing isn't too hard - take a look at the XFS
> buffer cache for an existing version.
> 
> It would be good to just to do the right thing inside bio_map_kern
> for that so that callers don't need to care if it is vmalloced or
> not.

Yes. That would help. I know md also needs to manually add pages on
vmalloced memory. Probably other do too.

> 
>> Ok. So this would mean that targets (e.g., pblk) deal with struct
>> request instead of only dealing with bios and then letting the LightNVM
>> core transforming bios to requests. This way we can directly map to the
>> request. Is this what you mean?
> 
> Yes.
> 
>> Just out of curiosity, why is forming the bio trough bio_copy_kern (or
>> manually doing the same) and then transforming to a request incorrect /
>> worse?
> 
> Because you expose yourself to the details of mapping a bio to request.
> We had to export blk_init_request_from_bio just for lightnvm to do this,
> and it also has to do weird other bits about requests.  If you go
> through blk_rq_map_* the block layer takes care of all that for you.

Ok. It makes sense. I'll talk to Matias about it.


Thanks!
Javier

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