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SubjectRe: [PATCH] fs/proc: smaps should avoid VM_PFNMAP areas
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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