linux-kernel - Re: [PATCH v5 0/6] Introduce ZONE

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Message-ID: <20160901060142.GA25364@js1304-P5Q-DELUXE>
Date:   Thu, 1 Sep 2016 15:01:43 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Laura Abbott <lauraa@...eaurora.org>,
        Minchan Kim <minchan@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 0/6] Introduce ZONE_CMA

On Thu, Sep 01, 2016 at 11:17:23AM +0530, Aneesh Kumar K.V wrote:
> Joonsoo Kim <iamjoonsoo.kim@....com> writes:
> 
> > On Tue, Aug 30, 2016 at 04:09:37PM +0530, Aneesh Kumar K.V wrote:
> >> Joonsoo Kim <js1304@...il.com> writes:
> >> 
> >> > 2016-08-29 18:27 GMT+09:00 Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>:
> >> >> js1304@...il.com writes:
> >> >>
> >> >>> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> >> >>>
> >> >>> 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.