Messages in this thread | | | Date | Tue, 21 Jan 2014 15:24:08 -0800 (PST) | From | David Rientjes <> | Subject | Re: [BUG] mm: thp: hugepage_vma_check has a blind spot |
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On Tue, 21 Jan 2014, Alex Thorlton wrote:
> hugepage_vma_check is called during khugepaged_scan_mm_slot to ensure > that khugepaged doesn't try to allocate THPs in vmas where they are > disallowed, either due to THPs being disabled system-wide, or through > MADV_NOHUGEPAGE. > > The logic that hugepage_vma_check uses doesn't seem to cover all cases, > in my opinion. Looking at the original code: > > if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) || > (vma->vm_flags & VM_NOHUGEPAGE)) > > We can see that it's possible to have THP disabled system-wide, but still > receive THPs in this vma. It seems that it's assumed that just because > khugepaged_always == false, TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG must be > set, which is not the case. We could have VM_HUGEPAGE set, but have THP > set to "never" system-wide, in which case, the condition presented in the > if will evaluate to false, and (provided the other checks pass) we can > end up giving out a THP even though the behavior is set to "never." >
You should be able to add a
BUG_ON(current != khugepaged_thread);
here since khugepaged is supposed to be the only caller to the function.
> While we do properly check these flags in khugepaged_has_work, it looks > like it's possible to sleep after we check khugepaged_hask_work, but > before hugepage_vma_check, during which time, hugepages could have been > disabled system-wide, in which case, we could hand out THPs when we > shouldn't be. >
You're talking about when thp is set to "never" and before khugepaged has stopped, correct?
That doesn't seem like a bug to me or anything that needs to be fixed, the sysfs knob could be switched even after hugepage_vma_check() is called and before a hugepage is actually collapsed so you have the same race.
The only thing that's guaranteed is that, upon writing "never" to /sys/kernel/mm/transparent_hugepage/enabled, no more thp memory will be collapsed after khugepaged has stopped.
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