Re: [PATCH v4 2/3] mm: Add support for kmem caches in DMA32 zone

[Nicolas Boichat <drinkcat@chromium.org>]

On Wed, Dec 5, 2018 at 5:56 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Wed 05-12-18 13:48:27, Nicolas Boichat wrote:
> > In some cases (e.g. IOMMU ARMv7s page allocator), we need to allocate
> > data structures smaller than a page with GFP_DMA32 flag.
> >
> > This change makes it possible to create a custom cache in DMA32 zone
> > using kmem_cache_create, then allocate memory using kmem_cache_alloc.
> >
> > We do not create a DMA32 kmalloc cache array, as there are currently
> > no users of kmalloc(..., GFP_DMA32). The new test in check_slab_flags
> > ensures that such calls still fail (as they do before this change).
>
> The changelog should be much more specific about decisions made here.
> First of all it would be nice to mention the usecase.

Ok, I'll copy most of the cover letter text here (i.e. the fact that
IOMMU wants physical memory <4GB for L2 page tables, why it's better
than genalloc/page_frag).

> Secondly, why do we need a new sysfs file? Who is going to consume it?

We have cache_dma, so it seems consistent to add cache_dma32.

I wasn't aware of tools/vm/slabinfo.c, so I can add support for
cache_dma32 in a follow-up patch. Any other user I should take care
of?

> Then why do we need SLAB_MERGE_SAME to cover GFP_DMA32 as well?

SLAB_MERGE_SAME tells us which flags _need_ to be the same for the
slabs to be merged. We don't want slab caches with GFP_DMA32 and
~GFP_DMA32 to be merged, so it should be in there.
(https://elixir.bootlin.com/linux/v4.19.6/source/mm/slab_common.c#L342).

> I
> thought the whole point is to use dedicated slab cache. Who is this
> going to merge with?

Well, if there was another SLAB cache requiring 1KB GFP_DMA32
elements, then I don't see why we would not merge the caches. This is
what happens with this IOMMU L2 tables cache pre-CONFIG_ZONE_DMA32 on
arm64 (output on some 3.18 kernel below), and what would happen on
arm32 since we still use GFP_DMA.

/sys/kernel/slab # ls -l | grep dt-0001024
drwxr-xr-x. 2 root root 0 Dec  5 02:25 :dt-0001024
lrwxrwxrwx. 1 root root 0 Dec  5 02:25 dma-kmalloc-1024 -> :dt-0001024
lrwxrwxrwx. 1 root root 0 Dec  5 02:25 io-pgtable_armv7s_l2 -> :dt-0001024

Thanks!

> --
> Michal Hocko
> SUSE Labs