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Date: Mon, 29 Apr 2013 21:03:08 -0400
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
To: Cliff Wickman <cpw@sgi.com>
Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, mgorman@suse.de,
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Message-ID: <1367283788-6nb3mvy8-mutt-n-horiguchi@ah.jp.nec.com>
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References: <E1UWva5-0005fY-Km@eag09.americas.sgi.com>
Subject: Re: [PATCH] fs/proc: smaps should avoid VM_PFNMAP areas
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Hello Cliff,

On Mon, Apr 29, 2013 at 04:25:53PM -0500, Cliff Wickman wrote:
> 
> /proc/<pid>/smaps should not be looking at VM_PFNMAP areas.
> 
> Certain tests in show_smap() (especially for huge pages) assume that the
> mapped PFN's are backed with page structures.  And this is not usually true
> for VM_PFNMAP areas.  This can result in panics on kernel page faults when
> attempting to address those page structures.

I think it's strange that you mention to hugepages, because in my understanding
VM_PFNMAP and hugepage related vma (VM_HUGEPAGE or VM_HUGETLB) should not set
at the same time. In what testcase are these flags both set?

And I guess this race can also happen on reading pagemap or numa_maps because
walk_page_range() is called in those code paths. Are you sure the race doesn't
happen on these paths? If not, please add a few more flag checks for them.

Thanks,
Naoya Horiguchi

> VM_PFNMAP areas are used by
> - graphics memory manager   gpu/drm/drm_gem.c
> - global reference unit     sgi-gru/grufile.c
> - sgi special memory        char/mspec.c
> - probably several out-of-tree modules
> 
> I'm copying everyone who has changed fs/proc/task_mmu.c recently, in case
> of some reason to provide /proc/<pid>/smaps for these areas that I am not
> aware of.
> 
> Signed-off-by: Cliff Wickman <cpw@sgi.com>
> ---
>  fs/proc/task_mmu.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> Index: linux/fs/proc/task_mmu.c
> ===================================================================
> --- linux.orig/fs/proc/task_mmu.c
> +++ linux/fs/proc/task_mmu.c
> @@ -589,6 +589,9 @@ static int show_smap(struct seq_file *m,
>  		.private = &mss,
>  	};
>  
> +	if (vma->vm_flags & VM_PFNMAP)
> +		return 0;
> +
>  	memset(&mss, 0, sizeof mss);
>  	mss.vma = vma;
>  	/* mmap_sem is held in m_start */
> --
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