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

[Nicolas Boichat <drinkcat@chromium.org>]

On Sun, Nov 11, 2018 at 5:04 PM Nicolas Boichat <drinkcat@chromium.org> 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.

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

Thanks,

Nicolas