Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 4 Dec 2018 16:28:44 +0000
From:   Will Deacon <will.deacon@arm.com>
To:     Nicolas Boichat <drinkcat@chromium.org>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        Christoph Lameter <cl@linux.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>,
        Matthias Brugger <matthias.bgg@gmail.com>, hch@infradead.org,
        Matthew Wilcox <willy@infradead.org>,
        Joerg Roedel <joro@8bytes.org>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Levin Alexander <Alexander.Levin@microsoft.com>,
        Huaisheng Ye <yehs1@lenovo.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>,
        iommu@lists.linux-foundation.org,
        lkml <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Yong Wu <yong.wu@mediatek.com>, Tomasz Figa <tfiga@google.com>,
        yingjoe.chen@mediatek.com, Hsin-Yi Wang <hsinyi@chromium.org>,
        Daniel Kurtz <djkurtz@chromium.org>
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>
 <CANMq1KDxmRcWhtaJbrLHqx6yPGkNaK7WNYYf+iFjH1e8XdrwRg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CANMq1KDxmRcWhtaJbrLHqx6yPGkNaK7WNYYf+iFjH1e8XdrwRg@mail.gmail.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@chromium.org> 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