Re: [PATCH v5 0/6] Introduce ZONE_CMA

[Joonsoo Kim]

On Thu, Sep 01, 2016 at 11:17:23AM +0530, Aneesh Kumar K.V wrote:
> Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> writes:
> 
> > On Tue, Aug 30, 2016 at 04:09:37PM +0530, Aneesh Kumar K.V wrote:
> >> Joonsoo Kim <js1304@xxxxxxxxx> writes:
> >> 
> >> > 2016-08-29 18:27 GMT+09:00 Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>:
> >> >> js1304@xxxxxxxxx writes:
> >> >>
> >> >>> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> >> >>>
> >> >>> Hello,
> >> >>>
> >> >>> Changes from v4
> >> >>> o Rebase on next-20160825
> >> >>> o Add general fix patch for lowmem reserve
> >> >>> o Fix lowmem reserve ratio
> >> >>> o Fix zone span optimizaion per Vlastimil
> >> >>> o Fix pageset initialization
> >> >>> o Change invocation timing on cma_init_reserved_areas()
> >> >>
> >> >> I don't see much information regarding how we interleave between
> >> >> ZONE_CMA and other zones for movable allocation. Is that explained in
> >> >> any of the patch ? The fair zone allocator got removed by
> >> >> e6cbd7f2efb433d717af72aa8510a9db6f7a7e05
> >> >
> >> > Interleaving would not work since the fair zone allocator policy is removed.
> >> > I don't think that it's a big problem because it is just matter of
> >> > timing to fill
> >> > up the memory. Eventually, memory on ZONE_CMA will be fully used in
> >> > any case.
> >> 
> >> Does that mean a CMA allocation will now be slower because in most case we
> >> will need to reclaim ? The zone list will now have ZONE_CMA in the
> >> beginning right ?
> >
> > ZONE_CMA will be used first but I don't think that CMA allocation will
> > be slower. In most case, memory would be fully used (usually
> > by page cache). So, we need reclaim or migration in any case.
> 
> Considering that the upstream kernel doesn't allow migration of THP
> pages, this would mean that migrate will fail in most case if we have
> THP enabled and the THP allocation request got satisfied via ZONE_CMA.
> Isn't that going to be a problem ?

I think that it is a separate problem. Once we restore utilization of
CMA area, it would become a problem in any case. It is just hidden by
utilization bug and should be handled separately. I guess that it's
not that hard to fix that problem.

Thanks.