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

[Will Deacon <will.deacon@arm.com>]

On Tue, Dec 04, 2018 at 05:37:13PM +0800, Nicolas Boichat wrote:
> 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.

FWIW, I'm open to any solution at this point, since I'd like to see this
regression fixed. (1) does sound better longer-term, but (3) looks pretty
much ready to do afaict.

Will