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From: Vlastimil Babka <vbabka@suse.cz>
To: Nicolas Boichat <drinkcat@chromium.org>
Cc: robin.murphy@arm.com, will.deacon@arm.com, joro@8bytes.org,
	cl@linux.com, penberg@kernel.org, rientjes@google.com,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	mhocko@suse.com, mgorman@techsingularity.net, yehs1@lenovo.com,
	rppt@linux.vnet.ibm.com,
	linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>,
	iommu@lists.linux-foundation.org,
	lkml <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, yong.wu@mediatek.com,
	Matthias Brugger <matthias.bgg@gmail.com>,
	tfiga@google.com, yingjoe.chen@mediatek.com,
	Alexander.Levin@microsoft.com
Subject: Re: [PATCH RFC 1/3] mm: When CONFIG_ZONE_DMA32 is set, use DMA32 for SLAB_CACHE_DMA
Date: Fri, 9 Nov 2018 13:14:41 +0100	[thread overview]
Message-ID: <8f58b778-f8ef-d32e-8803-2a20f171a0b1@suse.cz> (raw)
In-Reply-To: <CANMq1KB84Lpe_QbiuaKaBOwSsYr9Cis-gv5xpXaV5qjU=ON=7w@mail.gmail.com>

On 11/9/18 12:57 PM, Nicolas Boichat wrote:
> On Fri, Nov 9, 2018 at 6:43 PM Vlastimil Babka <vbabka@suse.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,
> 


  reply	other threads:[~2018-11-09 12:14 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-09  8:24 [PATCH RFC 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables Nicolas Boichat
2018-11-09  8:24 ` [PATCH RFC 1/3] mm: When CONFIG_ZONE_DMA32 is set, use DMA32 for SLAB_CACHE_DMA Nicolas Boichat
2018-11-09 10:43   ` Vlastimil Babka
2018-11-09 11:57     ` Nicolas Boichat
2018-11-09 12:14       ` Vlastimil Babka [this message]
2018-11-09  8:24 ` [PATCH RFC 2/3] include/linux/gfp.h: Add __get_dma32_pages macro Nicolas Boichat
2018-11-09  8:24 ` [PATCH RFC 3/3] iommu/io-pgtable-arm-v7s: Request DMA32 memory, and improve debugging Nicolas Boichat

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