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

[Nicolas Boichat]

On Tue, Dec 4, 2018 at 10:35 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 12/4/18 10:37 AM, Nicolas Boichat wrote:
> > On Sun, Nov 11, 2018 at 5:04 PM Nicolas Boichat <drinkcat@xxxxxxxxxxxx> wrote:
> >>
> >> This is a follow-up to the discussion in [1], to make sure that the page
> >> tables allocated by iommu/io-pgtable-arm-v7s are contained within 32-bit
> >> physical address space.
> >>
> >> [1] https://lists.linuxfoundation.org/pipermail/iommu/2018-November/030876.html
> >
> > Hi everyone,
> >
> > Let's try to summarize here.
> >
> > First, we confirmed that this is a regression, and IOMMU errors happen
> > on 4.19 and linux-next/master on MT8173 (elm, Acer Chromebook R13).
> > The issue most likely starts from ad67f5a6545f ("arm64: replace
> > ZONE_DMA with ZONE_DMA32"), i.e. 4.15, and presumably breaks a number
> > of Mediatek platforms (and maybe others?).
> >
> > We have a few options here:
> > 1. This series [2], that adds support for GFP_DMA32 slab caches,
> > _without_ adding kmalloc caches (since there are no users of
> > kmalloc(..., GFP_DMA32)). I think I've addressed all the comments on
> > the 3 patches, and AFAICT this solution works fine.
> > 2. genalloc. That works, but unless we preallocate 4MB for L2 tables
> > (which is wasteful as we usually only need a handful of L2 tables),
> > we'll need changes in the core (use GFP_ATOMIC) to allow allocating on
> > demand, and as it stands we'd have no way to shrink the allocation.
> > 3. page_frag [3]. That works fine, and the code is quite simple. One
> > drawback is that fragments in partially freed pages cannot be reused
> > (from limited experiments, I see that IOMMU L2 tables are rarely
> > freed, so it's unlikely a whole page would get freed). But given the
> > low number of L2 tables, maybe we can live with that.
> >
> > I think 2 is out. Any preference between 1 and 3? I think 1 makes
> > better use of the memory, so that'd be my preference. But I'm probably
> > missing something.
>
> I would prefer 1 as well. IIRC you already confirmed that alignment
> requirements are not broken for custom kmem caches even in presence of
> SLUB debug options (and I would say it's a bug to be fixed if they
> weren't).

> I just asked (and didn't get a reply I think) about your
> ability to handle the GFP_ATOMIC allocation failures. They should be
> rare when only single page allocations are needed for the kmem cache.
> But in case they are not an option, then preallocating would be needed,
> thus probably option 2.

Oh, sorry, I missed your question.

I don't have a full answer, but:
- The allocations themselves are rare (I count a few 10s of L2 tables
at most on my system, I assume we rarely have >100), and yes, we only
need a single page, so the failures should be exceptional.
- My change is probably not making anything worse: I assume that even
with the current approach using GFP_DMA slab caches on older kernels,
failures could potentially happen. I don't think we've seen those. If
we are really concerned about this, maybe we'd need to modify
mtk_iommu_map to not hold a spinlock (if that's possible), so we don't
need to use GFP_ATOMIC. I suggest we just keep an eye on such issues,
and address them if they show up (we can even revisit genalloc at that
stage).

Anyway, I'll clean up patches for 1 (mostly commit message changes
based on the comments in the threads) and resend.

Thanks,

> > [2] https://patchwork.kernel.org/cover/10677529/, 3 patches
> > [3] https://patchwork.codeaurora.org/patch/671639/
> >
> > Thanks,
> >
> > Nicolas
> >
>