linux-kernel - Re: [PATCH 01/10] swiotlb-xen: use vmalloc_to

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Message-ID: <alpine.DEB.2.21.2005211235590.27502@sstabellini-ThinkPad-T480s>
Date:   Thu, 21 May 2020 20:54:47 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Julien Grall <julien@....org>
cc:     Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>
Subject: Re: [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@...cle.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@...cle.com>
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.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?