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.

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.

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

> 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.

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

needs to use bio_add_page.

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

needs to use bio_add_page.

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