Re: [PATCH] base: dma-mapping: Postpone cpu addr translation on mmap()

[jacopo mondi]

Hi Christoph,

On Mon, Apr 09, 2018 at 10:52:51AM -0700, Christoph Hellwig wrote:
> On Mon, Apr 09, 2018 at 06:59:08PM +0200, Jacopo Mondi wrote:
> > I'm still a bit puzzled on what happens if dma_mmap_from_dev_coherent() fails.
> > Does a dma_mmap_from_dev_coherent() failure guarantee anyhow that the
> > successive virt_to_page() isn't problematic as it is today?
> > Or is it the
> >  	if (off < count && user_count <= (count - off))
> > check that makes the translation safe?
>
> It doesn't.  I think one major issue is that we should not simply fall
> to dma_common_mmap if no method is required, but need every instance of
> dma_map_ops to explicitly opt into an mmap method that is known to work.

I see.. this patch thus just postpones the problem...

>
> >  #ifndef CONFIG_ARCH_NO_COHERENT_DMA_MMAP
> >  	unsigned long user_count = vma_pages(vma);
> >  	unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
> > -	unsigned long pfn = page_to_pfn(virt_to_page(cpu_addr));
> >  	unsigned long off = vma->vm_pgoff;
> > +	unsigned long pfn;
> >
> >  	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> >
> > @@ -235,6 +235,7 @@ int dma_common_mmap(struct device *dev, struct vm_area_struct *vma,
> >  		return ret;
> >
> >  	if (off < count && user_count <= (count - off)) {
> > +		pfn = page_to_pfn(virt_to_page(cpu_addr));
> >  		ret = remap_pfn_range(vma, vma->vm_start,
> >  				      pfn + off,
> >  				      user_count << PAGE_SHIFT,
>
> Why not:
>
> 		ret = remap_pfn_range(vma, vma->vm_start,
> 				page_to_pfn(virt_to_page(cpu_addr)) + off,
>
> and save the temp variable?

Sure, it's better... Should I send a v2 or considering your above
comment this patch is just a mitigation and should be ditched in
favour of a proper solution (which requires a much more considerable amount
of work though)?

Thanks
   j

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