Messages in this thread | | | Date | Tue, 30 Apr 2013 13:52:02 -0500 | From | Cliff Wickman <> | Subject | Re: [PATCH] fs/proc: smaps should avoid VM_PFNMAP areas |
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> 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 structures.
Then a sequence like: show_smap show_map_vma walk_page_range walk_pud_range walk_pmd_range split_huge_page_pmd(walk->mm, pmd) __split_huge_page_pmd page = pmd_page(*pmd) can address (vmemmap + (pfn)) and panic
Or a sequence like this: walk_pmd_range walk->pmd_entry(pmd, addr, next, walk) smaps_pte_range smaps_pte_entry(*pte, addr, PAGE_SIZE, walk) page = vm_normal_page(vma, addr, ptent) return pfn_to_page(pfn) > > 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.
-Cliff Wickman
> 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 */
-- Cliff Wickman SGI cpw@sgi.com (651) 683-3824
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