linux-kernel - Re: [Xen-devel] [PATCH 1/3] xen/swiotlb: fix condition for calling xen_destroy_contiguous

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Date:   Thu, 25 Apr 2019 02:53:41 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "Stefano Stabellini" <sstabellini@...nel.org>,
        <iommu@...ts.linux-foundation.org>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 1/3] xen/swiotlb: fix condition for
 calling xen_destroy_contiguous_region()

>>> On 23.04.19 at 12:54, <jgross@...e.com> 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