Subject: Re: [PATCH] powerpc/crashdump : fix page frame number check in copy_oldmem_page

From: Mahesh J Salgaonkar <[email-masked]>
Date: Thu, 27 Feb 2014 00:08:48 +0530

On 2014-02-24 17:30:55 Mon, Laurent Dufour wrote:
> In copy_oldmem_page, the current check using max_pfn and min_low_pfn to
> decide if the page is backed or not, is not valid when the memory layout is
> not continuous.
> 
> This happens when running as a QEMU/KVM guest, where RTAS is mapped higher
> in the memory. In that case max_pfn points to the end of RTAS, and a hole
> between the end of the kdump kernel and RTAS is not backed by PTEs. As a
> consequence, the kdump kernel is crashing in copy_oldmem_page when accessing
> in a direct way the pages in that hole.
> 
> This fix relies on the memblock's service memblock_is_region_memory to
> check if the read page is part or not of the directly accessible memory.
> 
> Signed-off-by: Laurent Dufour <[email-masked]>

Tested on PowerNV (BE), where without this patch we see cp and 
makedumpfile fails with "Bad address" while reading /proc/vmcore.
With this patch makedumpfile and cp succeeds.

Tested-by: Mahesh Salgaonkar <[email-masked]>

> ---
>  arch/powerpc/kernel/crash_dump.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/crash_dump.c b/arch/powerpc/kernel/crash_dump.c
> index 11c1d06..7a13f37 100644
> --- a/arch/powerpc/kernel/crash_dump.c
> +++ b/arch/powerpc/kernel/crash_dump.c
> @@ -98,17 +98,19 @@ ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
>  			size_t csize, unsigned long offset, int userbuf)
>  {
>  	void  *vaddr;
> +	phys_addr_t paddr;
>  
>  	if (!csize)
>  		return 0;
>  
>  	csize = min_t(size_t, csize, PAGE_SIZE);
> +	paddr = pfn << PAGE_SHIFT;
>  
> -	if ((min_low_pfn < pfn) && (pfn < max_pfn)) {
> -		vaddr = __va(pfn << PAGE_SHIFT);
> +	if (memblock_is_region_memory(paddr, csize)) {
> +		vaddr = __va(paddr);
>  		csize = copy_oldmem_vaddr(vaddr, buf, csize, offset, userbuf);
>  	} else {
> -		vaddr = __ioremap(pfn << PAGE_SHIFT, PAGE_SIZE, 0);
> +		vaddr = __ioremap(paddr, PAGE_SIZE, 0);
>  		csize = copy_oldmem_vaddr(vaddr, buf, csize, offset, userbuf);
>  		iounmap(vaddr);
>  	}
> 
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-- 
Mahesh J Salgaonkar