Re: [Xen-devel] [PATCH 1/3] xen/swiotlb: fix condition for calling xen_destroy_contiguous_region()

[Juergen Gross]

On 25/04/2019 10:53, Jan Beulich wrote:
>>>> On 23.04.19 at 12:54, <jgross@xxxxxxxx> wrote:
>> --- a/drivers/xen/swiotlb-xen.c
>> +++ b/drivers/xen/swiotlb-xen.c
>> @@ -360,8 +360,8 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
>>  	/* Convert the size to actually allocated. */
>>  	size = 1UL << (order + XEN_PAGE_SHIFT);
>>  
>> -	if (((dev_addr + size - 1 <= dma_mask)) ||
>> -	    range_straddles_page_boundary(phys, size))
>> +	if ((dev_addr + size - 1 <= dma_mask) &&
>> +	    !WARN_ON(range_straddles_page_boundary(phys, size)))
>>  		xen_destroy_contiguous_region(phys, order);
> 
> On the allocation side we have
> 
> 	if (((dev_addr + size - 1 <= dma_mask)) &&
> 	    !range_straddles_page_boundary(phys, size))
> 		*dma_handle = dev_addr;
> 	else {
> 		if (xen_create_contiguous_region(phys, order,
> 						 fls64(dma_mask), dma_handle) != 0) {
> 			xen_free_coherent_pages(hwdev, size, ret, (dma_addr_t)phys, attrs);
> 			return NULL;
> 		}
> 	}
> 
> which is (as far as the function call is concerned)
> 
> 	if ((dev_addr + size - 1 > dma_mask) ||
> 	    range_straddles_page_boundary(phys, size))
> 		xen_create_contiguous_region(...);
> 
> So I don't think your transformation is correct. Even worse, both
> parts of the condition in xen_swiotlb_free_coherent() act on an
> address that is the _result_ of the prior
> xen_create_contiguous_region(), i.e. the address should always
> match _both_ criteria anyway. Whereas what you really want is
> undo the xen_create_contiguous_region() only when it actually
> was called. Otherwise you also shatter contiguous allocations
> that were contiguous already for other reasons (perhaps just
> luck).

Yes, that is what patch 3 does.


Juergen