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From:   Nicolas Boichat <drinkcat@chromium.org>
Date:   Thu, 22 Nov 2018 13:56:47 +0800
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Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables
To:     willy@infradead.org
Cc:     Robin Murphy <robin.murphy@arm.com>,
        Christoph Lameter <cl@linux.com>,
        Will Deacon <will.deacon@arm.com>,
        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>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>,
        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>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Tomasz Figa <tfiga@google.com>, yingjoe.chen@mediatek.com
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On Thu, Nov 22, 2018 at 10:36 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Nov 21, 2018 at 10:26:26PM +0000, Robin Murphy wrote:
> > These are IOMMU page tables, rather than CPU ones, so we're already well
> > outside arch code - indeed the original motivation of io-pgtable was to be
> > entirely independent of the p*d types and arch-specific MM code (this Armv7
> > short-descriptor format is already "non-native" when used by drivers in an
> > arm64 kernel).
>
> There was quite a lot of explanation missing from this patch description!

I totally agree ,-) I'm not familiar at all with either iommu or
mm/... Looks like the patchset triggered a helpful discussion, and I
understand the problem better now. I'll improve the description in the
next revision.

> > There are various efficiency reasons for using regular kernel memory instead
> > of coherent DMA allocations - for the most part it works well, we just have
> > the odd corner case like this one where the 32-bit format gets used on
> > 64-bit systems such that the tables themselves still need to be allocated
> > below 4GB (although the final output address can point at higher memory by
> > virtue of the IOMMU in question not implementing permissions and repurposing
> > some of those PTE fields as extra address bits).
> >
> > TBH, if this DMA32 stuff is going to be contentious we could possibly just
> > rip out the offending kmem_cache - it seemed like good practice for the
> > use-case, but provided kzalloc(SZ_1K, gfp | GFP_DMA32) can be relied upon to
> > give the same 1KB alignment and chance of succeeding as the equivalent
> > kmem_cache_alloc(), then we could quite easily make do with that instead.
>
> I think you should look at using the page_frag allocator here.  You can
> use whatever GFP_DMA flags you like.

I'll try that.

Thanks!