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

[Christoph Hellwig]

On Wed, Sep 06, 2017 at 04:00:56PM +0200, Javier González 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.

> 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.