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

[Nicolas Boichat <drinkcat@chromium.org>]

On Thu, Nov 22, 2018 at 10:36 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Nov 21, 2018 at 10:26:26PM +0000, Robin Murphy wrote:
> > These are IOMMU page tables, rather than CPU ones, so we're already well
> > outside arch code - indeed the original motivation of io-pgtable was to be
> > entirely independent of the p*d types and arch-specific MM code (this Armv7
> > short-descriptor format is already "non-native" when used by drivers in an
> > arm64 kernel).
>
> There was quite a lot of explanation missing from this patch description!

I totally agree ,-) I'm not familiar at all with either iommu or
mm/... Looks like the patchset triggered a helpful discussion, and I
understand the problem better now. I'll improve the description in the
next revision.

> > There are various efficiency reasons for using regular kernel memory instead
> > of coherent DMA allocations - for the most part it works well, we just have
> > the odd corner case like this one where the 32-bit format gets used on
> > 64-bit systems such that the tables themselves still need to be allocated
> > below 4GB (although the final output address can point at higher memory by
> > virtue of the IOMMU in question not implementing permissions and repurposing
> > some of those PTE fields as extra address bits).
> >
> > TBH, if this DMA32 stuff is going to be contentious we could possibly just
> > rip out the offending kmem_cache - it seemed like good practice for the
> > use-case, but provided kzalloc(SZ_1K, gfp | GFP_DMA32) can be relied upon to
> > give the same 1KB alignment and chance of succeeding as the equivalent
> > kmem_cache_alloc(), then we could quite easily make do with that instead.
>
> I think you should look at using the page_frag allocator here.  You can
> use whatever GFP_DMA flags you like.

I'll try that.

Thanks!