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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

Jan



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