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

[Vlastimil Babka]

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.