linux-kernel - Re: could you clarify mm/mempolicy: fix !vma in new_vma

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Date:	Tue, 7 Jan 2014 09:34:25 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>, Bob Liu <lliubbo@...il.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Bob Liu <bob.liu@...cle.com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: could you clarify mm/mempolicy: fix !vma in new_vma_page()

On Tue 07-01-14 12:34:34, Wanpeng Li wrote:
> Cced Sasha,
> On Tue, Jan 07, 2014 at 12:26:13PM +0800, Wanpeng Li wrote:
> >Hi Michal,
> >On Mon, Jan 06, 2014 at 03:18:27PM +0100, Michal Hocko wrote:
> >>On Mon 06-01-14 20:45:54, Bob Liu wrote:
> >>[...]
> >>>  544         if (PageAnon(page)) {
> >>>  545                 struct anon_vma *page__anon_vma = page_anon_vma(page);
> >>>  546                 /*
> >>>  547                  * Note: swapoff's unuse_vma() is more efficient with this
> >>>  548                  * check, and needs it to match anon_vma when KSM is active.
> >>>  549                  */
> >>>  550                 if (!vma->anon_vma || !page__anon_vma ||
> >>>  551                     vma->anon_vma->root != page__anon_vma->root)
> >>>  552                         return -EFAULT;
> >>>  553         } else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
> >>>  554                 if (!vma->vm_file ||
> >>>  555                     vma->vm_file->f_mapping != page->mapping)
> >>>  556                         return -EFAULT;
> >>>  557         } else
> >>>  558                 return -EFAULT;
> >>> 
> >>> That's the "other conditions" and the reason why we can't use
> >>> BUG_ON(!vma) in new_vma_page().
> >>
> >>Sorry, I wasn't clear with my question. I was interested in which of
> >>these triggered and why only for hugetlb pages?
> >
> >Not just for hugetlb pages, sorry for do two things in one patch. The change 
> >for hugetlb pages is to fix the potential dereference NULL pointer reported 
> >by Dan. http://marc.info/?l=linux-mm&m=137689530323257&w=2 
> >
> >If we should ask Sasha to add more debug information to dump which condition 
> >is failed in page_address_in_vma() for you?

I am always more calm when the removed BUG_ON is properly understood and
justified.
-- 
Michal Hocko
SUSE Labs
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