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Message-ID: <c5036587af69e70a60f6187048be1991bb2d7e7b.camel@mediatek.com>
Subject: Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING
From:   Walter Wu <walter-zh.wu@mediatek.com>
To:     Christoph Hellwig <hch@lst.de>
CC:     Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        <iommu@lists.linux-foundation.org>, <linux-kernel@vger.kernel.org>,
        <linux-arm-kernel@lists.infradead.org>,
        wsd_upstream <wsd_upstream@mediatek.com>,
        <linux-mediatek@lists.infradead.org>,
        "Andrew Morton" <akpm@linux-foundation.org>
Date:   Tue, 2 Nov 2021 15:26:12 +0800
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Hi Christoph,

I am sorry, fix my typo.


Walter

On Tue, 2021-11-02 at 07:41 +0100, Christoph Hellwig wrote:
> As others pointed out, DMA_ATTR_NO_KERNEL_MAPPING just means the
> caller can't rely on a kernel mapping.  So the "fix" here is
> wrong.  That being said for cases where we can easily remove a page
> from the kernel mapping it would be nice to do to:
> 
>  a) improve security
>  b) as a debug check to see that no one actually tries to access it
> 
> > +		/* remove kernel mapping for pages */
> > +		set_memory_valid((unsigned
> > long)phys_to_virt(dma_to_phys(dev, *dma_handle)),
> 
> Please avoid overly long lines.  Also this function only exists for
> arm64
> also and others pointed out won't work for all cases.