Re: [PATCH] fs/proc: smaps should avoid VM_PFNMAP areas
From: Cliff Wickman
Date: Tue Apr 30 2013 - 14:52:10 EST
> On Tue, Apr 30, 2013 at 01:11:45PM -0500, Naoya Horiguchi 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?
I don't think VM_PFNMAP and VM_HUGE... set at the same time.
The problem is that a VM_PFNMAP'd area might have 2MB mappings in its
page table, but they may point to pfn's that are not backed by page
Then a sequence like:
page = pmd_page(*pmd)
can address (vmemmap + (pfn)) and panic
Or a sequence like this:
walk->pmd_entry(pmd, addr, next, walk)
smaps_pte_entry(*pte, addr, PAGE_SIZE, walk)
page = vm_normal_page(vma, addr, ptent)
> 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.
Okay. I'll check and submit a version 2 of this patch.
> 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@xxxxxxx>
> > ---
> > 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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