linux-kernel - Re: [BUG] BUG: unable to handle kernel paging request at fffba000

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Date:	Thu, 20 Jan 2011 00:49:50 +0200
From:	Ilya Dryomov <idryomov@...il.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>,
	linux-kernel@...r.kernel.org, idryomov@...il.com
Subject: Re: [BUG] BUG: unable to handle kernel paging request at fffba000

On Wed, Jan 19, 2011 at 11:19:09PM +0100, Andrea Arcangeli wrote:
> Hello Ilya,
> 
> thanks for sending me the gdb info too.
> 
> can you test this fix? Thanks a lot! (it only affected x86 32bit
> builds with highpte enabled)
> 
> ====
> Subject: fix pte_unmap in khugepaged for highpte x86_32
> 
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> __collapse_huge_page_copy is still dereferencing the pte passed as parameter so
> we must pte_unmap after __collapse_huge_page_copy returns, not before.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>

It fixes the above problem for me.  Thanks a lot Andrea.

> ---
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 004c9c2..c4f634b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1837,9 +1837,9 @@ static void collapse_huge_page(struct mm_struct *mm,
>  	spin_lock(ptl);
>  	isolated = __collapse_huge_page_isolate(vma, address, pte);
>  	spin_unlock(ptl);
> -	pte_unmap(pte);
>  
>  	if (unlikely(!isolated)) {
> +		pte_unmap(pte);
>  		spin_lock(&mm->page_table_lock);
>  		BUG_ON(!pmd_none(*pmd));
>  		set_pmd_at(mm, address, pmd, _pmd);
> @@ -1856,6 +1856,7 @@ static void collapse_huge_page(struct mm_struct *mm,
>  	anon_vma_unlock(vma->anon_vma);
>  
>  	__collapse_huge_page_copy(pte, new_page, vma, address, ptl);
> +	pte_unmap(pte);
>  	__SetPageUptodate(new_page);
>  	pgtable = pmd_pgtable(_pmd);
>  	VM_BUG_ON(page_count(pgtable) != 1);
> 
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