linux-kernel - Re: [PATCH 1/3] mm/cma: manage the memory of the CMA area by using the ZONE

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Date:   Thu, 31 Aug 2017 10:40:49 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Vlastimil Babka <vbabka@...e.cz>
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>,
        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 1/3] mm/cma: manage the memory of the CMA area by using
 the ZONE_MOVABLE

On Tue, Aug 29, 2017 at 11:16:18AM +0200, Vlastimil Babka wrote:
> On 08/24/2017 08:36 AM, js1304@...il.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > 0. History
> > 
> > This patchset is the follow-up of the discussion about the
> > "Introduce ZONE_CMA (v7)" [1]. Please reference it if more information
> > is needed.
> > 
> 
> [...]
> 
> > 
> > [1]: lkml.kernel.org/r/1491880640-9944-1-git-send-email-iamjoonsoo.kim@....com
> > [2]: https://lkml.org/lkml/2014/10/15/623
> > [3]: http://www.spinics.net/lists/linux-mm/msg100562.html
> > 
> > Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> > Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> The previous version has introduced ZONE_CMA, so I would think switching
> to ZONE_MOVABLE is enough to drop previous reviews. Perhaps most of the
> code involved is basically the same, though?

Yes, most of the code involved is the same. I considered to drop
previous review tags but most of the code and concept is the same so I
decide to keep review tags. I should mention it in cover-letter but I
forgot to mention it. Sorry about that.

> Anyway I checked the current patch and did some basic tests with qemu,
> so you can keep my ack.

Thanks!

> 
> BTW, if we dropped NR_FREE_CMA_PAGES, could we also drop MIGRATE_CMA and
> related hooks? Is that counter really that useful as it works right now?
> It will decrease both by CMA allocations (which has to be explicitly
> freed) and by movable allocations (which can be migrated). What if only
> CMA alloc/release touched it?

I think that NR_FREE_CMA_PAGES would not be as useful as previous. We
can remove it.

However, removing MIGRATE_CMA has a problem. There is an usecase to
check if the page comes from the CMA area or not. See
check_page_span() in mm/usercopy.c. I can implement it differently by
iterating whole CMA area and finding the match, but I'm not sure it's
performance effect. I guess that it would be marginal.

Anyway, I'd like not to cause any side-effect now. After patches are
settle down on mainline, I will try to remove them as you suggested.

Thanks.