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. [2] https://patchwork.kernel.org/cover/10677529/, 3 patches [3] https://patchwork.codeaurora.org/patch/671639/ Thanks, Nicolas
