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Date:   Wed, 5 Dec 2018 12:37:22 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Nicolas Boichat <drinkcat@chromium.org>
Cc:     Will Deacon <will.deacon@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Joerg Roedel <joro@8bytes.org>,
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        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
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Subject: Re: [PATCH v4 2/3] mm: Add support for kmem caches in DMA32 zone
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On Wed 05-12-18 19:01:03, Nicolas Boichat wrote:
[...]
> > Secondly, why do we need a new sysfs file? Who is going to consume it?
> 
> We have cache_dma, so it seems consistent to add cache_dma32.

I wouldn't copy a pattern unless there is an explicit usecase for it.
We do expose way too much to userspace and that keeps kicking us later.
Not that I am aware of any specific example for cache_dma but seeing
other examples I would rather be more careful.

> I wasn't aware of tools/vm/slabinfo.c, so I can add support for
> cache_dma32 in a follow-up patch. Any other user I should take care
> of?

In general zones are inernal MM implementation details and the less we
export to userspace the better.

> > Then why do we need SLAB_MERGE_SAME to cover GFP_DMA32 as well?
> 
> SLAB_MERGE_SAME tells us which flags _need_ to be the same for the
> slabs to be merged. We don't want slab caches with GFP_DMA32 and
> ~GFP_DMA32 to be merged, so it should be in there.
> (https://elixir.bootlin.com/linux/v4.19.6/source/mm/slab_common.c#L342).

Ohh, my bad, I have misread the change. Sure we definitely not want to
allow merging here. My bad.
-- 
Michal Hocko
SUSE Labs