On Thu, Nov 22, 2018 at 10:36 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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!
