Re: [Lsf-pc] [LSF/MM ATTEND] block: multipage bvecs

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sun, 2016-02-28 at 13:17 +0200, Boaz Harrosh wrote:
> On 02/26/2016 06:33 PM, Ming Lei wrote:
> > Hi,
> > 
> > I'd like to participate in LSF/MM and discuss multipage bvecs.
> > 
> > Kent posted the idea[1] before, but never pushed out.
> > I have studied multipage bvecs for a while, and think
> > it is a good idea to improve block subsystem.
> > 
> > Multipage bvecs means that one 'struct bio_bvec' can hold
> > multiple pages which are physically contiguous instead
> > of one single page used in current kernel.
> > 
> 
> Hi Ming Lei
> 
> This is an interesting talk for me.
> 
> I don't know if you ever tried it but I did. If I take a regular
> SSD disk or a PCIE flash card that I have in my machine and
> I stick a pointer to a page and bv_len = PAGE_SIZE * 8 and call
> submit_bio, I get 8 pages worth of IO with a single bvec and it
> all just works.

No it wouldn't.  There's no check anywhere that a single bv entry is
under the queue max segment size because the assumption is bv_len <=
page size.  If you start sending multi-page single bio vec entries,
you'll violate those assumptions and eventually get an unmappable bio.

James