linux-kernel - Re: [PATCH v4 2/3] mm: Add support for kmem caches in DMA32 zone

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Date:   Wed, 5 Dec 2018 12:37:22 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     Will Deacon <will.deacon@....com>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Levin Alexander <Alexander.Levin@...rosoft.com>,
        Huaisheng Ye <yehs1@...ovo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        iommu@...ts.linux-foundation.org,
        lkml <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Yong Wu <yong.wu@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Tomasz Figa <tfiga@...gle.com>, yingjoe.chen@...iatek.com,
        hch@...radead.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 2/3] mm: Add support for kmem caches in DMA32 zone

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