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Subject: Re: mm: BUG in khugepaged_scan_mm_slot
To: Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, Hugh Dickins <hughd@google.com>,
        Greg Thelen <gthelen@google.com>,
        Konstantin Khlebnikov <koct9i@gmail.com>
References: <CACT4Y+ZmuZMV5CjSFOeXviwQdABAgT7T+StKfTqan9YDtgEi5g@mail.gmail.com>
Cc: syzkaller <syzkaller@googlegroups.com>, Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        Andrea Arcangeli <aarcange@redhat.com>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <5702582A.1030904@suse.cz>
Date: Mon, 4 Apr 2016 14:03:54 +0200
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[+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.