linux-kernel - Re: [PATCH RFC 1/3] mm: When CONFIG_ZONE_DMA32 is set, use DMA32 for SLAB_CACHE

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Date:   Fri, 9 Nov 2018 13:14:41 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     robin.murphy@....com, will.deacon@....com, joro@...tes.org,
        cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, mhocko@...e.com,
        mgorman@...hsingularity.net, yehs1@...ovo.com,
        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@...iatek.com, Matthias Brugger <matthias.bgg@...il.com>,
        tfiga@...gle.com, yingjoe.chen@...iatek.com,
        Alexander.Levin@...rosoft.com
Subject: Re: [PATCH RFC 1/3] mm: When CONFIG_ZONE_DMA32 is set, use DMA32 for
 SLAB_CACHE_DMA

On 11/9/18 12:57 PM, Nicolas Boichat wrote:
> On Fri, Nov 9, 2018 at 6:43 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>> Also I'm probably missing the point of this all. In patch 3 you use
>> __get_dma32_pages() thus __get_free_pages(__GFP_DMA32), which uses
>> alloc_pages, thus the page allocator directly, and there's no slab
>> caches involved.
> 
> __get_dma32_pages fixes level 1 page allocations in the patch 3.
> 
> This change fixes level 2 page allocations
> (kmem_cache_zalloc(data->l2_tables, gfp | GFP_DMA)), by transparently
> remapping GFP_DMA to an underlying ZONE_DMA32.
> 
> The alternative would be to create a new SLAB_CACHE_DMA32 when
> CONFIG_ZONE_DMA32 is defined, but then I'm concerned that the callers
> would need to choose between the 2 (GFP_DMA or GFP_DMA32...), and also
> need to use some ifdefs (but maybe that's not a valid concern?).
> 
>> It makes little sense to involve slab for page table
>> allocations anyway, as those tend to be aligned to a page size (or
>> high-order page size). So what am I missing?
> 
> Level 2 tables are ARM_V7S_TABLE_SIZE(2) => 1kb, so we'd waste 3kb if
> we allocated a full page.

Oh, I see.

Well, I think indeed the most transparent would be to support
SLAB_CACHE_DMA32. The callers of kmem_cache_zalloc() would then need not
add anything special to gfp, as that's stored internally upon
kmem_cache_create(). Of course SLAB_BUG_MASK would no longer have to
treat __GFP_DMA32 as unexpected. It would be unexpected when passed to
kmalloc() which doesn't have special dma32 caches, but for a cache
explicitly created to allocate from ZONE_DMA32, I don't see why not. I'm
somewhat surprised that there wouldn't be a need for this earlier, so
maybe I'm still missing something.

> Thanks,
>