linux-kernel - Re: [GIT PULL] memremap fix for 4.3 (v2)

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Date:	Sun, 1 Nov 2015 20:36:56 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	"Williams, Dan J" <dan.j.williams@...el.com>
Cc:	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>
Subject: Re: [GIT PULL] memremap fix for 4.3 (v2)

On Sun, Nov 01, 2015 at 08:24:25PM +0000, Williams, Dan J wrote:
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 72b0c66628b6..9d6b55587eaa 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -24,6 +24,16 @@ __weak void __iomem *ioremap_cache(resource_size_t offset, unsigned long size)
>  }
>  #endif
>  
> +static void *try_ram_remap(resource_size_t offset, size_t size)
> +{
> +	struct page *page = pfn_to_page(offset >> PAGE_SHIFT);
> +
> +	/* In the simple case just return the existing linear address */
> +	if (!PageHighMem(page))
> +		return __va(offset);
> +	return NULL; /* fallback to ioremap_cache */

Right, so a highmem page results in this returning NULL, which will...

> @@ -66,8 +76,8 @@ void *memremap(resource_size_t offset, size_t size, unsigned long flags)
>  		 * the requested range is potentially in "System RAM"
>  		 */
>  		if (is_ram == REGION_INTERSECTS)
> -			addr = __va(offset);
> -		else
> +			addr = try_ram_remap(offset, size);
> +		if (!addr)
>  			addr = ioremap_cache(offset, size);

cause ioremap_cache() to be called.  That falls through on ARM to:

	__arm_ioremap_pfn_caller()

and we will hit this (as I've mentioned before through explanation
rather than quoting code):

        /*
         * Don't allow RAM to be mapped - this causes problems with ARMv6+
         */
        if (WARN_ON(pfn_valid(pfn)))
                return NULL;

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