From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables Date: Wed, 5 Dec 2018 14:41:30 +0000 Message-ID: <20181205144130.GA16121@arm.com> References: <20181111090341.120786-1-drinkcat@chromium.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Nicolas Boichat Cc: Michal Hocko , Daniel Kurtz , Levin Alexander , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Christoph Lameter , Huaisheng Ye , Matthew Wilcox , linux-arm Mailing List , David Rientjes , yingjoe.chen-NuS5LvNUpcJWk0Htik3J/w@public.gmane.org, Vlastimil Babka , Tomasz Figa , Mike Rapoport , Hsin-Yi Wang , Matthias Brugger , Joonsoo Kim , Robin Murphy , lkml , Pekka Enberg , iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Andrew Morton , Mel Gorman List-Id: iommu@lists.linux-foundation.org On Wed, Dec 05, 2018 at 10:04:00AM +0800, Nicolas Boichat wrote: > On Tue, Dec 4, 2018 at 10:35 PM Vlastimil Babka wrote: > > > > On 12/4/18 10:37 AM, Nicolas Boichat wrote: > > > On Sun, Nov 11, 2018 at 5:04 PM 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. > > >> > > >> [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). I think the spinlock is the least of our worries: the map/unmap routines can be called in irq context and may need to allocate second-level tables. Will