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SubjectRe: [PATCH 01/10] swiotlb-xen: use vmalloc_to_page on vmalloc virt addresses
On Thu, 21 May 2020, Julien Grall wrote:
> Hi,
>
> On 21/05/2020 00:45, Stefano Stabellini wrote:
> > From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> >
> > Don't just assume that virt_to_page works on all virtual addresses.
> > Instead add a is_vmalloc_addr check and use vmalloc_to_page on vmalloc
> > virt addresses.
>
> Can you provide an example where swiotlb is used with vmalloc()?

The issue was reported here happening on the Rasperry Pi 4:
https://marc.info/?l=xen-devel&m=158862573216800

If you are asking where in the Linux codebase the vmalloc is happening
specifically, I don't know for sure, my information is limited to the
stack trace that you see in the link (I don't have a Rasperry Pi 4 yet
but I shall have one soon.)


> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
> > ---
> > drivers/xen/swiotlb-xen.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index b6d27762c6f8..a42129cba36e 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -335,6 +335,7 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t
> > size, void *vaddr,
> > int order = get_order(size);
> > phys_addr_t phys;
> > u64 dma_mask = DMA_BIT_MASK(32);
> > + struct page *pg;
> > if (hwdev && hwdev->coherent_dma_mask)
> > dma_mask = hwdev->coherent_dma_mask;
> > @@ -346,9 +347,11 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t
> > size, void *vaddr,
> > /* Convert the size to actually allocated. */
> > size = 1UL << (order + XEN_PAGE_SHIFT);
> > + pg = is_vmalloc_addr(vaddr) ? vmalloc_to_page(vaddr) :
> > + virt_to_page(vaddr);
>
> Common DMA code seems to protect this check with CONFIG_DMA_REMAP. Is it
> something we want to do it here as well? Or is there any other condition where
> vmalloc can happen?

I can see it in dma_direct_free_pages:

if (IS_ENABLED(CONFIG_DMA_REMAP) && is_vmalloc_addr(cpu_addr))
vunmap(cpu_addr);

I wonder why the common DMA code does that. is_vmalloc_addr should work
regardless of CONFIG_DMA_REMAP. Maybe just for efficiency?

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