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Date:   Thu, 31 Aug 2017 13:32:12 +0200
From:   Vlastimil Babka <>
To:     Joonsoo Kim <>
Cc:     Andrew Morton <>,
        Rik van Riel <>,
        Johannes Weiner <>,, Laura Abbott <>,
        Minchan Kim <>,
        Marek Szyprowski <>,
        Michal Nazarewicz <>,
        "Aneesh Kumar K . V" <>,
        Russell King <>,
        Will Deacon <>,,,,
        Kees Cook <>
Subject: Re: [PATCH 1/3] mm/cma: manage the memory of the CMA area by using

On 08/31/2017 03:40 AM, Joonsoo Kim wrote:
> On Tue, Aug 29, 2017 at 11:16:18AM +0200, Vlastimil Babka wrote:
>> On 08/24/2017 08:36 AM, wrote:
>>> From: Joonsoo Kim <>
>>> 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]:
>>> [2]:
>>> [3]:
>>> Reviewed-by: Aneesh Kumar K.V <>
>>> Acked-by: Vlastimil Babka <>
>> 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.

+CC Kees Cook

Hmm, seems like this check is to make sure we don't copy from/to parts
of kernel memory we're not supposed to? Then I believe checking that
pages are in ZONE_MOVABLE should then give the same guarantees as

BTW the comment says "Reject if range is entirely either Reserved or
CMA" but the code does the opposite thing. I assume the comment is wrong?

> 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.
> --
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