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Date: Mon, 26 Nov 2018 00:02:13 -0800
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page
 tables
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To: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Hellwig <hch@infradead.org>, Robin Murphy <robin.murphy@arm.com>, Matthew Wilcox <willy@infradead.org>, Christopher Lameter <cl@linux.com>, Levin Alexander <Alexander.Levin@microsoft.com>, Mike Rapoport <rppt@linux.vnet.ibm.com>, Nicolas Boichat <drinkcat@chromium.org>, Huaisheng Ye <yehs1@lenovo.com>, Tomasz Figa <tfiga@google.com>, Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org, Pekka Enberg <penberg@kernel.org>, linux-mm@kvack.org, iommu@lists.linux-foundation.org, Michal Hocko <mhocko@suse.com>, linux-arm-kernel@lists.infradead.org, David Rientjes <rientjes@google.com>, Matthias Brugger <matthias.bgg@gmail.com>, yingjoe.chen@mediatek.com, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Mel Gorman <mgorman@techsingularity.net>, Andrew Morton <akpm@linux-foundation.org>

On Fri, Nov 23, 2018 at 01:23:41PM +0100, Vlastimil Babka wrote:
> Is this also true for caches created by kmem_cache_create(), that
> debugging options can result in not respecting the alignment passed to
> kmem_cache_create()? That would be rather bad, IMHO.

That's what I understood in the discussion.  If not it would make
our live simpler, but would need to be well document.

Christoph can probably explain the alignment choices in slub.

> 
> > But I do agree with the sentiment of not wanting to spread GFP_DMA32
> > futher into the slab allocator.
> 
> I don't see a problem with GFP_DMA32 for custom caches. Generic
> kmalloc() would be worse, since it would have to create a new array of
> kmalloc caches. But that's already ruled out due to the alignment.

True, purely slab probably isn't too bad.