Re: mm: BUG in khugepaged_scan_mm_slot

From: Vlastimil Babka
Date: Mon Apr 04 2016 - 08:04:34 EST


[+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.