Re: mm: BUG in khugepaged_scan_mm_slot

[Kirill A. Shutemov]

On Mon, Apr 04, 2016 at 02:03:54PM +0200, Vlastimil Babka wrote:
> [+CC Andrea]
> 
> On 04/02/2016 11:48 AM, Dmitry Vyukov wrote:
> >Hello,
> >
> >The following program triggers a BUG in khugepaged_scan_mm_slot:
> >
> >
> >vma ffff880032698f90 start 0000000020c57000 end 0000000020c58000
> >next ffff88003269a1b8 prev ffff88003269ac18 mm ffff88005e274780
> >prot 35 anon_vma ffff88003182c000 vm_ops           (null)
> >pgoff fed00 file ffff8800324552c0 private_data           (null)
> >flags: 0x5144477(read|write|exec|mayread|maywrite|mayexec|pfnmap|io|dontexpand|account)
> >------------[ cut here ]------------
> >kernel BUG at mm/huge_memory.c:2313!
> >invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
> 
> That's VM_BUG_ON_VMA(vma->vm_flags & VM_NO_THP, vma) in
> hugepage_vma_check().
> 
> #define VM_NO_THP (VM_SPECIAL | VM_HUGETLB | VM_SHARED | VM_MAYSHARE)
> 
> #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)
> 
> Of those, we have VM_IO | VM_DONTEXPAND.
> 
> I don't know if it's valid for a vma with anon_vma to have such flags, if
> yes, we should probably modify hugepage_vma_check(). Called from
> khugepaged_scan_mm_slot() it should just return false out VM_NO_THP. Called
> from collapse_huge_page() it could keep the VM_BUG_ON. Or maybe just have
> VM_BUG_ON(!hugepage_vma_check()) there? Hmm actually no, there's a mmap_sem
> release for read and then acquire for write, so we can't rely on the check
> done earlier from khugepaged_scan_mm_slot().
> 
> So we should probably just change the VM_BUG_ON to another "return false"
> condition. Unless the VM_BUG_ON uncovered a real bug and the earlier
> conditions in hugepage_vma_check() should guarantee the VM_BUG_ON be false
> for any vma.

145961146490.28194.16019687861681349309.stgit@zurg">http://lkml.kernel.org/r/145961146490.28194.16019687861681349309.stgit@zurg

-- 
 Kirill A. Shutemov