Re: GFP_REPEAT usage in vhost_net_open resp. vhost_vsock_dev_open

[Michal Hocko <mhocko@kernel.org>]

On Wed 04-01-17 19:56:42, Michael S. Tsirkin wrote:
> On Wed, Jan 04, 2017 at 04:08:00PM +0100, Michal Hocko wrote:
> > Hi Michael,
> > I am currently cleaning up opencoded kmalloc with vmalloc fallback users
> > [1] and my current kvmalloc_node helper doesn't support GFP_REPEAT
> > because there are no users which would need it. At least that's what I
> > thought until I've encountered vhost_vsock_dev_open resp.
> > vhost_vsock_dev_open which are trying to use GFP_REPEAT for kmalloc.
> > 23cc5a991c7a ("vhost-net: extend device allocation to vmalloc") explains
> > the motivation as follows:
> > "
> > As vmalloc() adds overhead on a critical network path, add __GFP_REPEAT
> > to kzalloc() flags to do this fallback only when really needed.
> > "
> > 
> > I am wondering whether vmalloc adds more overhead than GFP_REPEAT
> 
> Yes but the GFP_REPEAT overhead is during allocation time.
> Using vmalloc means all accesses are slowed down.
> Allocation is not on data path, accesses are.

OK, that wasn't clear to me. Thanks for the clarification. If the access
path can compensate the allocation cost then I agree that GFP_REPEAT
makes a lot of sense. I will cook up a patch to allow GFP_REPEAT in the
current kvmalloc_node and convert vhost users to it.

Thanks!

-- 
Michal Hocko
SUSE Labs

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