linux-kernel - Re: [Xen-devel] [PATCH v2 3/3] xen/swiotlb: remember having called xen_create_contiguous

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Date:   Wed, 29 May 2019 03:57:37 -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>
Subject: Re: [Xen-devel] [PATCH v2 3/3] xen/swiotlb: remember having
 called xen_create_contiguous_region()

>>> On 29.05.19 at 11:04, <jgross@...e.com> wrote:
> @@ -345,8 +346,11 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
>  	size = 1UL << (order + XEN_PAGE_SHIFT);
>  
>  	if (!WARN_ON((dev_addr + size - 1 > dma_mask) ||
> -		     range_straddles_page_boundary(phys, size)))
> +		     range_straddles_page_boundary(phys, size)) &&
> +	    PageXenRemapped(virt_to_page(vaddr))) {
>  		xen_destroy_contiguous_region(phys, order);
> +		ClearPageXenRemapped(virt_to_page(vaddr));
> +	}

To be symmetric with setting the flag only after having made the region
contiguous, and to avoid (perhaps just theoretical) races, wouldn't it be
better to clear the flag before calling xen_destroy_contiguous_region()?
Even better would be a TestAndClear...() operation.

Jan