On Tue, Jan 21, 2014 at 03:24:08PM -0800, David Rientjes wrote: > 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? Yes, that's 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. That makes sense, I wasn't aware that that's the expected behavior here. I suppose this isn't something that needs to be changed, in that case. I needed the logic broken out a bit more explicitly (madvise/never case need to be handled separately) for a patch that I'm working on - that's when this caught my attention. Good to know that a change to the system-wide switch shouldn't affect khugepaged if it's already running. I would've screwed up that behavior with my patch :) Thanks, David! - Alex -- 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>