From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi1-f197.google.com (mail-oi1-f197.google.com [209.85.167.197]) by kanga.kvack.org (Postfix) with ESMTP id B1F626B6F9E for ; Tue, 4 Dec 2018 11:28:27 -0500 (EST) Received: by mail-oi1-f197.google.com with SMTP id h85so10761393oib.9 for ; Tue, 04 Dec 2018 08:28:27 -0800 (PST) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id y132si7530374oig.260.2018.12.04.08.28.25 for ; Tue, 04 Dec 2018 08:28:25 -0800 (PST) Date: Tue, 4 Dec 2018 16:28:44 +0000 From: Will Deacon Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables Message-ID: <20181204162844.GA8169@arm.com> References: <20181111090341.120786-1-drinkcat@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Nicolas Boichat Cc: Robin Murphy , Christoph Lameter , Vlastimil Babka , Michal Hocko , Matthias Brugger , hch@infradead.org, Matthew Wilcox , Joerg Roedel , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Mel Gorman , Levin Alexander , Huaisheng Ye , Mike Rapoport , linux-arm Mailing List , iommu@lists.linux-foundation.org, lkml , linux-mm@kvack.org, Yong Wu , Tomasz Figa , yingjoe.chen@mediatek.com, Hsin-Yi Wang , Daniel Kurtz On Tue, Dec 04, 2018 at 05:37:13PM +0800, 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. 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