linux-kernel - Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables

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Date:   Fri, 23 Nov 2018 13:23:41 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christoph Hellwig <hch@...radead.org>,
        Robin Murphy <robin.murphy@....com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        Levin Alexander <Alexander.Levin@...rosoft.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Huaisheng Ye <yehs1@...ovo.com>,
        Tomasz Figa <tfiga@...gle.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Michal Hocko <mhocko@...e.com>,
        linux-arm-kernel@...ts.infradead.org,
        David Rientjes <rientjes@...gle.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        yingjoe.chen@...iatek.com, Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page
 tables

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.