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

[Nicolas Boichat]

On Mon, Nov 26, 2018 at 4:02 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> 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.