Re: [PATCH v3 07/49] bcache: comment on direct access to bvec table

[Ming Lei]

On Thu, Aug 10, 2017 at 04:26:03AM -0700, Christoph Hellwig wrote:
> I think all this bcache code needs bigger attention.  For one
> bio_alloc_pages is only used in bcache, so we should move it in there.

Looks a good idea.

> 
> Second the way  bio_alloc_pages is currently written looks potentially
> dangerous for multi-page biovecs, so we should think about a better
> calling convention.  The way bcache seems to generally use it is by
> allocating a bio, then calling bch_bio_map on it and then calling
> bio_alloc_pages.  I think it just needs a new bio_alloc_pages calling
> convention that passes the size to be allocated and stop looking into
> the segment count.

Looks a good idea, will try to do in this way.

> 
> Second bch_bio_map isn't something we should be doing in a driver,
> it should be rewritten using bio_add_page.

Yes, the idea way is to use bio_add_page always, but given
bch_bio_map() is used on a fresh bio, it is safe, and this
work can be done in another bcache cleanup patch.

> 
> > diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> > index 866dcf78ff8e..3da595ae565b 100644
> > --- a/drivers/md/bcache/btree.c
> > +++ b/drivers/md/bcache/btree.c
> > @@ -431,6 +431,7 @@ static void do_btree_node_write(struct btree *b)
> >  
> >  		continue_at(cl, btree_node_write_done, NULL);
> >  	} else {
> > +		/* No harm for multipage bvec since the new is just allocated */
> >  		b->bio->bi_vcnt = 0;
> 
> This should go away - bio_alloc_pages or it's replacement should not
> modify bi_vcnt on failure.

OK.

> 
> > +	/* single page bio, safe for multipage bvec */
> >  	dc->sb_bio.bi_io_vec[0].bv_page = sb_page;
> 
> needs to use bio_add_page.

OK.

> 
> > +	/* single page bio, safe for multipage bvec */
> >  	ca->sb_bio.bi_io_vec[0].bv_page = sb_page;
> 
> needs to use bio_add_page.

OK.

-- 
Ming