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Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page
 tables
To:     Christoph Hellwig <hch@infradead.org>,
        Robin Murphy <robin.murphy@arm.com>
Cc:     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>
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From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Fri, 23 Nov 2018 13:23:41 +0100
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On 11/22/18 9:23 AM, Christoph Hellwig wrote:
> On Wed, Nov 21, 2018 at 10:26:26PM +0000, Robin Murphy wrote:
>> TBH, if this DMA32 stuff is going to be contentious we could possibly just
>> rip out the offending kmem_cache - it seemed like good practice for the
>> use-case, but provided kzalloc(SZ_1K, gfp | GFP_DMA32) can be relied upon to
>> give the same 1KB alignment and chance of succeeding as the equivalent
>> kmem_cache_alloc(), then we could quite easily make do with that instead.
> 
> Neither is the slab support for kmalloc, not do kmalloc allocations
> have useful alignment apparently (at least if you use slub debug).

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.

> 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.

> I think you want a simple genalloc allocator for this rather special
> use case.

I would prefer if slab could support it, as it doesn't have to
preallocate. OTOH if the allocations are GFP_ATOMIC as suggested later
in the thread, and need to always succeed, then preallocation could be
better, and thus maybe genalloc.