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From: Nicolas Boichat <drinkcat@chromium.org>
Date: Wed, 28 Nov 2018 16:55:54 +0800
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Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables
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Cc: Vlastimil Babka <vbabka@suse.cz>, Robin Murphy <robin.murphy@arm.com>, willy@infradead.org, Christoph Lameter <cl@linux.com>, Levin Alexander <Alexander.Levin@microsoft.com>, Mike Rapoport <rppt@linux.vnet.ibm.com>, Huaisheng Ye <yehs1@lenovo.com>, Tomasz Figa <tfiga@google.com>, Will Deacon <will.deacon@arm.com>, lkml <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 Mailing List <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 Mon, Nov 26, 2018 at 4:02 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> 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.

>>From my experiment, adding `slub_debug` to command line does _not_
break the alignment of kmem_cache_alloc'ed objects.

We do see an increase in slab_size
(/sys/kernel/slab/io-pgtable_armv7s_l2/slab_size), from 1024 to 3072
(probably because slub needs to allocate space on each side for the
red zone/padding, while keeping the alignment?)

> 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.