linux-kernel - Re: Regression in v4.2-rc1: vmalloc_to

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Date:	Tue, 21 Jul 2015 14:43:38 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Ashutosh Dixit <ashutosh.dixit@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Dutt, Sudeep" <sudeep.dutt@...el.com>,
	"Rao, Nikhil" <nikhil.rao@...el.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: Regression in v4.2-rc1: vmalloc_to_page with ioremap

On Tue, 2015-07-21 at 14:39 -0600, Toshi Kani wrote:
> On Tue, 2015-07-21 at 08:17 -0700, Ashutosh Dixit wrote:
> > On Mon, Jul 20 2015 at 12:21:18 PM, Toshi Kani <toshi.kani@...com>

 :
> > Yes, you are correct, the 'struct page' pointer returned by
> > vmalloc_to_page does not point to a "real" struct page entry. Neither
> > have we allocated a struct page for the range. However, because we pass
> > the returned pointer to streaming DMA mapping API's
> > (dma_map_ops->dma_map_sg or dma_map_ops->dma_map_page) and all those
> > functions do is call page_to_phys, they only care about the physical
> > address, it used to work.
> > 
> > Would it be possible to have a different API which can do this or can
> > vmalloc_to_page be updated to handle huge ioremaps without crashing? 
> >  Or
> > would you have a suggestion for doing this differently?
> 
> You can do the following instead.  If you have the physical address 
> already
> (i.e. the address you passed to ioremap), you can skip 
> slow_virt_to_phys().
>  pfn_to_page() is a hack for the time being so that you can use the same
> DMA mapping APIs.
> 
>   phys = slow_virt_to_phys(vaddr);
>   page = pfn_to_page(phys >> PAGE_SHIFT);

Forgot to mention.  slow_virt_to_phys() is only defined in x86, but I think
your driver is x86-only.  Let me know if this is a problem.

Thanks,
-Toshi

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