linux-kernel - Re: [PATCH v3] dma-debug: fix incorrect pfn calculation

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Date:   Sun, 1 Oct 2017 10:04:49 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     miles.chen@...iatek.com, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        wsd_upstream@...iatek.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v3] dma-debug: fix incorrect pfn calculation

On Wed, Sep 27, 2017 at 11:23:52AM +0100, Robin Murphy wrote:
> > I found that debug_dma_alloc_coherent() and debug_dma_free_coherent()
> > assume that dma_alloc_coherent() always returns a linear address.
> > However it's possible that dma_alloc_coherent() returns a non-linear
> > address. In this case, page_to_pfn(virt_to_page(virt)) will return an
> > incorrect pfn. If the pfn is valid and mapped as a COW page,
> > we will hit the warning when doing wp_page_copy().

Hmm, can the debug code assume anything?  Right now you're just patching
it from supporting linear and vmalloc.  But what about other
potential mapping types?

> > +	entry->pfn	 = is_vmalloc_addr(virt) ? vmalloc_to_pfn(virt) :
> > +						page_to_pfn(virt_to_page(virt));

Please use normal if/else conditionsals:

	if (is_vmalloc_addr(virt))
		entry->pfn = vmalloc_to_pfn(virt);
	else
		entry->pfn = page_to_pfn(virt_to_page(virt));

> > +		.pfn		= is_vmalloc_addr(virt) ? vmalloc_to_pfn(virt) :
> > +						page_to_pfn(virt_to_page(virt)),

Same here.