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

[Will Deacon]

On Wed, Dec 05, 2018 at 10:04:00AM +0800, Nicolas Boichat wrote:
> 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).

I think the spinlock is the least of our worries: the map/unmap routines
can be called in irq context and may need to allocate second-level tables.

Will