Re: [PATCH net-next V4 0/5] vhost: accelerate metadata access through vmap()

[Jason Wang]

On 2019/1/27 äå8:31, Michael S. Tsirkin wrote:
> On Sat, Jan 26, 2019 at 02:37:08PM -0800, David Miller wrote:
> > From: Jason Wang <jasowang@xxxxxxxxxx>
> > Date: Wed, 23 Jan 2019 17:55:52 +0800
> >
> > > This series tries to access virtqueue metadata through kernel virtual
> > > address instead of copy_user() friends since they had too much
> > > overheads like checks, spec barriers or even hardware feature
> > > toggling.
> > >
> > > Test shows about 24% improvement on TX PPS. It should benefit other
> > > cases as well.
> > I've read over the discussion of patch #5 a few times.
> >
> > And it seems to me that, at a minimum, a few things still need to
> > be resolved:
> >
> > 1) More perf data added to commit message.


Ok.


> > 2) Whether invalidate_range_start() and invalidate_range_end() must
> >     be paired.


The reason that vhost doesn't need an invalidate_range_end() is because 
we have a fallback to copy_to_user() friends. So there's no requirement 
to setup the mapping in range_end() or lock the vq between range_start() 
and range_end(). We try to delay the setup of vmap until it will be 
really used in vhost_meta_prefetch() and we hold mmap_sem when trying to 
setup vmap, this will guarantee there's no intermediate state at this time.


> Add dirty tracking.


I think this could be solved by introducing e.g 
vhost_meta_prefetch_done() at the end of handle_tx()/handle_rx() and 
call set_page_dirty() for used pages instead of the tricks of 
classifying VMA. (As I saw hugetlbfs has its own set dirty method).

Thanks


> > Etc.  So I am marking this series "Changes Requested".