Re: mm: BUG in khugepaged_scan_mm_slot

[Vlastimil Babka]

[+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.