linux-kernel - Re: [PATCH v7 0/7] Introduce ZONE

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Message-ID: <20170517074424.GC18406@js1304-desktop>
Date:   Wed, 17 May 2017 16:44:26 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        mgorman@...hsingularity.net, Laura Abbott <lauraa@...eaurora.org>,
        Minchan Kim <minchan@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v7 0/7] Introduce ZONE_CMA

> > 
> > Okay. We did a lot of discussion so it's better to summarise it.
> > 
> > 1. ZONE_CMA might be a nicer solution than MIGRATETYPE.
> > 2. Additional bit in page flags would cause another kind of
> > maintenance problem so it's better to avoid it as much as possible.
> > 3. Abusing ZONE_MOVABLE looks better than introducing ZONE_CMA since
> > it doesn't need additional bit in page flag.
> > 4. (Not-yet-finished) If ZONE_CMA doesn't need extra bit in page
> > flags with hacky magic and it has no performance regression,
> > ??? (it's okay to use separate zone for CMA?)
> 
> As mentioned above. I do not see why we should go over additional hops
> just to have a zone which is not strictly needed. So if there are no
> inherent problems reusing MOVABLE/HIGMEM zone then a separate zone
> sounds like a wrong direction.
> 
> But let me repeat. I am _not_ convinced that the migratetype situation
> is all that bad and unfixable. You have mentioned some issues with the
> current approach but none of them seem inherently unfixable. So I would
> still prefer keeping the current way. But I am not going to insist if
> you _really_ believe that the long term maintenance cost will be higher
> than a zone approach and you can reuse MOVABLE/HIGHMEM zones without
> disruptive changes. I can help you with the hotplug part of the MOVABLE
> zone because that is desirable on its own.

Okay. Thanks for sharing your opinion. I will decide the final
direction after some investigation.

Thanks.