On Thu, Nov 22, 2018 at 6:27 AM Robin Murphy <robin.murphy@xxxxxxx> wrote: > > On 2018-11-21 9:38 pm, Matthew Wilcox wrote: > > On Wed, Nov 21, 2018 at 06:20:02PM +0000, Christopher Lameter wrote: > >> On Sun, 11 Nov 2018, Nicolas Boichat 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. > >> > >> Page tables? This means you need a page frame? Why go through the slab > >> allocators? > > > > Because this particular architecture has sub-page-size PMD page tables. > > We desperately need to hoist page table allocation out of the architectures; > > there're a bunch of different implementations and they're mostly bad, > > one way or another. > > 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 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. Yes, but if we want to use kzalloc, we'll need to create kmalloc_caches for DMA32, which seems wasteful as there are no other users (see my comment here: https://patchwork.kernel.org/patch/10677525/#22332697). Thanks, > Thanks, > Robin. > > > For each level of page table we generally have three cases: > > > > 1. single page > > 2. sub-page, naturally aligned > > 3. multiple pages, naturally aligned > > > > for 1 and 3, the page allocator will do just fine. > > for 2, we should have a per-MM page_frag allocator. s390 already has > > something like this, although it's more complicated. ppc also has > > something a little more complex for the cases when it's configured with > > a 64k page size but wants to use a 4k page table entry. > > > > I'd like x86 to be able to simply do: > > > > #define pte_alloc_one(mm, addr) page_alloc_table(mm, addr, 0) > > #define pmd_alloc_one(mm, addr) page_alloc_table(mm, addr, 0) > > #define pud_alloc_one(mm, addr) page_alloc_table(mm, addr, 0) > > #define p4d_alloc_one(mm, addr) page_alloc_table(mm, addr, 0) > > > > An architecture with 4k page size and needing a 16k PMD would do: > > > > #define pmd_alloc_one(mm, addr) page_alloc_table(mm, addr, 2) > > > > while an architecture with a 64k page size needing a 4k PTE would do: > > > > #define ARCH_PAGE_TABLE_FRAG > > #define pte_alloc_one(mm, addr) pagefrag_alloc_table(mm, addr, 4096) > > > > I haven't had time to work on this, but perhaps someone with a problem > > that needs fixing would like to, instead of burying yet another awful > > implementation away in arch/ somewhere. > >
