Hello, On Mon, Apr 04, 2016 at 03:06:25PM +0300, Kirill A. Shutemov wrote: > 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. > > http://lkml.kernel.org/r/145961146490.28194.16019687861681349309.stgit@zurg That's not the only place that assumes vm_ops NULL means anonymous and not VM_IO though, so I agree with Vlastimil we should think once more about this fix, either that or extend it to other places. I wonder if perhaps there was a mistake in checking vm_ops in the first place and leaving the vm_ops check isn't the right fix. Wouldn't it be more correct to apply a s/!vm_ops/!vm_file/ and not just there? What problem would then we run into if we used !vm_file? The assumption in this vm_ops check is that it was safer to a vm_file check but clearly it isn't as some chardev is not setting vm_ops (don't they need to vm_ops->close?). But all chardevs have vm_file set, so if we could use that instead, we can retain the VM_BUG_ON or better convert it to a graceful warn on that bails out. Thanks, Andrea -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>