Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP_SYNC capability

[Minchan Kim]

Hi Christoph,

On Fri, Aug 11, 2017 at 12:46:15PM +0200, Christoph Hellwig wrote:
> On Wed, Aug 09, 2017 at 08:06:24PM -0700, Dan Williams wrote:
> > I like it, but do you think we should switch to sbvec[<constant>] to
> > preclude pathological cases where nr_pages is large?
> 
> Yes, please.

Still, I don't understand how sbvec[nr_pages] with on-stack bio in
do_mpage_readpage can help the performance.

IIUC, do_mpage_readpage works with page-base. IOW, it passes just one
page, not multiple pages so if we use on-stack bio, we just add *a page*
via bio_add_page and submit the bio before the function returning.

So, rather than sbvec[1], why de we need sbvec[nr_pages]?

Please, let me open my eyes. :)

Thanks.