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

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Message-ID: <32d173a2-4b8c-909e-89fc-68ed9fb2fa02@suse.com>
Date:   Thu, 25 Apr 2019 11:03:02 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...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>,
        lkml <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 25/04/2019 10:53, Jan Beulich wrote:
>>>> 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.

I know that I modified the condition. Calling
xen_destroy_contiguous_region() for a non-contiguous region is
destructive, so we have to avoid it in any case.

I could be talked into moving both tests into the WARN_ON(), however.


Juergen