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

[Matthew Wilcox]

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!

> 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.