Subject: Re: [RESEND PATCH] powerpc/powernv: Fix vma page prot flags in opal-prd driver

From: Jeremy Kerr <[email-masked]>
Date: Mon, 29 Jun 2015 13:20:01 +0800

Hi Vaidy,

> opal-prd driver will mmap() firmware code/data area as private
> mapping to prd user space daemon.  Write to this page will
> trigger COW faults.  The new COW pages are normal kernel RAM
> pages accounted by the kernel and are not special.
> 
> vma->vm_page_prot value will be used at page fault time
> for the new COW pages, while pgprot_t value passed in
> remap_pfn_range() is used for the initial page table entry.
> 
> Hence:
> * Do not add _PAGE_SPECIAL in vma, but only for remap_pfn_range()
> * Also remap_pfn_range() will add the _PAGE_SPECIAL flag using
>   pte_mkspecial() call, hence no need to specify in the driver
> 
> This fix resolves the page accounting warning shown below:
> BUG: Bad rss-counter state mm:c0000007d34ac600 idx:1 val:19
> 
> The above warning is triggered since _PAGE_SPECIAL was incorrectly
> being set for the normal kernel COW pages.
> 
> Signed-off-by: Vaidyanathan Srinivasan <[email-masked]>

Acked-by: Jeremy Kerr <[email-masked]>

Cheers,

Jeremy