On Tue, Dec 04, 2018 at 05:37:13PM +0800, Nicolas Boichat wrote: > On Sun, Nov 11, 2018 at 5:04 PM Nicolas Boichat <drinkcat@xxxxxxxxxxxx> 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. > > > > [1] https://lists.linuxfoundation.org/pipermail/iommu/2018-November/030876.html > > Hi everyone, > > Let's try to summarize here. > > First, we confirmed that this is a regression, and IOMMU errors happen > on 4.19 and linux-next/master on MT8173 (elm, Acer Chromebook R13). > The issue most likely starts from ad67f5a6545f ("arm64: replace > ZONE_DMA with ZONE_DMA32"), i.e. 4.15, and presumably breaks a number > of Mediatek platforms (and maybe others?). > > We have a few options here: > 1. This series [2], that adds support for GFP_DMA32 slab caches, > _without_ adding kmalloc caches (since there are no users of > kmalloc(..., GFP_DMA32)). I think I've addressed all the comments on > the 3 patches, and AFAICT this solution works fine. > 2. genalloc. That works, but unless we preallocate 4MB for L2 tables > (which is wasteful as we usually only need a handful of L2 tables), > we'll need changes in the core (use GFP_ATOMIC) to allow allocating on > demand, and as it stands we'd have no way to shrink the allocation. > 3. page_frag [3]. That works fine, and the code is quite simple. One > drawback is that fragments in partially freed pages cannot be reused > (from limited experiments, I see that IOMMU L2 tables are rarely > freed, so it's unlikely a whole page would get freed). But given the > low number of L2 tables, maybe we can live with that. > > I think 2 is out. Any preference between 1 and 3? I think 1 makes > better use of the memory, so that'd be my preference. But I'm probably > missing something. FWIW, I'm open to any solution at this point, since I'd like to see this regression fixed. (1) does sound better longer-term, but (3) looks pretty much ready to do afaict. Will
