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.
