linux-kernel - Re: [PATCH 1/2] dma-mapping: remove bogus test for pfn_valid from dma_map

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Date:   Thu, 30 Sep 2021 07:46:37 +0200
From:   Christoph Hellwig <hch@....de>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Alex Bee <knaerzche@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Hellwig <hch@....de>,
        David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 1/2] dma-mapping: remove bogus test for pfn_valid from
 dma_map_resource

On Thu, Sep 30, 2021 at 04:30:38AM +0300, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> dma_map_resource() uses pfn_valid() to ensure the range is not RAM.
> However, pfn_valid() only checks for availability of the memory map for a
> PFN but it does not ensure that the PFN is actually backed by RAM.
> 
> As dma_map_resource() is the only method in DMA mapping APIs that has this
> check, simply drop the pfn_valid() test from dma_map_resource().
> 
> Link: https://lore.kernel.org/all/20210824173741.GC623@arm.com/
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>

We'll lose a bit of a sanity check with this, but given the problems it
caused:

Reviewed-by: Christoph Hellwig <hch@....de>