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 10:55:57 +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-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, 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 13:48:27, Nicolas Boichat wrote:
> In some cases (e.g. IOMMU ARMv7s page allocator), we need to allocate
> data structures smaller than a page with GFP_DMA32 flag.
> 
> This change makes it possible to create a custom cache in DMA32 zone
> using kmem_cache_create, then allocate memory using kmem_cache_alloc.
> 
> We do not create a DMA32 kmalloc cache array, as there are currently
> no users of kmalloc(..., GFP_DMA32). The new test in check_slab_flags
> ensures that such calls still fail (as they do before this change).

The changelog should be much more specific about decisions made here.
First of all it would be nice to mention the usecase.

Secondly, why do we need a new sysfs file? Who is going to consume it?

Then why do we need SLAB_MERGE_SAME to cover GFP_DMA32 as well? I
thought the whole point is to use dedicated slab cache. Who is this
going to merge with?
-- 
Michal Hocko
SUSE Labs