linux-kernel - Re: 2.6.38-rc1 problems with khugepaged

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Date:	Thu, 20 Jan 2011 01:01:47 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	werner <w.landgraf@...ru>, Borislav Petkov <bp@...64.org>,
	Ilya Dryomov <idryomov@...il.com>,
	linux-mm <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Subject: Re: 2.6.38-rc1 problems with khugepaged

On Wed, Jan 19, 2011 at 10:45:23PM +0100, Johannes Weiner wrote:
> Hi,
> 
> On Wed, Jan 19, 2011 at 04:59:54PM +0100, Borislav Petkov wrote:
> > Adding some more parties to CC.
> > 
> > On Wed, Jan 19, 2011 at 09:45:25AM -0400, werner wrote:
> > > **  Help   Help Help ***
> > > 
> > > My computer crashs on booting  ...   :( :(
> 
> That sucks!
> 
> I cross-compiled for 32-bit and was able to match up the disassembly
> against the code line from your oops report.  Apparently the pte was
> an invalid pointer, and it makes perfect sense: we unmap the highpte
> _before_ we access the pointer again for __collapse_huge_page_copy().
> 
> Can you test with this fix applied?  It is only compile-tested, I too
> have no 32-bit installations anymore.

Thanks Johannes, I already sent the same fix a few minutes ago, it is
also confirmed to work from Ilya in Message-ID:
<20110119224950.GA3429@...ngo.lan.net>

> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1be1034..e187454 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1839,9 +1839,9 @@ 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);
> @@ -1858,6 +1858,7 @@ 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);

Good thing your fix is identical to mine ;).
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