linux-kernel - Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables

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Date:   Wed, 21 Nov 2018 18:35:58 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christopher Lameter <cl@...ux.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Will Deacon <will.deacon@....com>,
        Joerg Roedel <joro@...tes.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Levin Alexander <Alexander.Levin@...rosoft.com>,
        Huaisheng Ye <yehs1@...ovo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Yong Wu <yong.wu@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Tomasz Figa <tfiga@...gle.com>, yingjoe.chen@...iatek.com
Subject: Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page
 tables

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!

> 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.