On Tue, Dec 4, 2018 at 10:35 PM Vlastimil Babka <vbabka@xxxxxxx> wrote: > > On 12/4/18 10:37 AM, 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. > > I would prefer 1 as well. IIRC you already confirmed that alignment > requirements are not broken for custom kmem caches even in presence of > SLUB debug options (and I would say it's a bug to be fixed if they > weren't). > I just asked (and didn't get a reply I think) about your > ability to handle the GFP_ATOMIC allocation failures. They should be > rare when only single page allocations are needed for the kmem cache. > But in case they are not an option, then preallocating would be needed, > thus probably option 2. Oh, sorry, I missed your question. I don't have a full answer, but: - The allocations themselves are rare (I count a few 10s of L2 tables at most on my system, I assume we rarely have >100), and yes, we only need a single page, so the failures should be exceptional. - My change is probably not making anything worse: I assume that even with the current approach using GFP_DMA slab caches on older kernels, failures could potentially happen. I don't think we've seen those. If we are really concerned about this, maybe we'd need to modify mtk_iommu_map to not hold a spinlock (if that's possible), so we don't need to use GFP_ATOMIC. I suggest we just keep an eye on such issues, and address them if they show up (we can even revisit genalloc at that stage). Anyway, I'll clean up patches for 1 (mostly commit message changes based on the comments in the threads) and resend. Thanks, > > [2] https://patchwork.kernel.org/cover/10677529/, 3 patches > > [3] https://patchwork.codeaurora.org/patch/671639/ > > > > Thanks, > > > > Nicolas > > >
