On 2016/7/21 15:43, Michal Hocko wrote: > We have further discussed the patch and I believe it is not correct. See [1]. > I am proposing the following alternative. > > [1] http://lkml.kernel.org/r/20160720132431.GM11249@xxxxxxxxxxxxxx > --- > >From b1e9b3214f1859fdf7d134cdcb56f5871933539c Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@xxxxxxxx> > Date: Thu, 21 Jul 2016 09:28:13 +0200 > Subject: [PATCH] mm, hugetlb: fix huge_pte_alloc BUG_ON > > Zhong Jiang has reported a BUG_ON from huge_pte_alloc hitting when he > runs his database load with memory online and offline running in > parallel. The reason is that huge_pmd_share might detect a shared pmd > which is currently migrated and so it has migration pte which is > !pte_huge. > > There doesn't seem to be any easy way to prevent from the race and in > fact seeing the migration swap entry is not harmful. Both callers of > huge_pte_alloc are prepared to handle them. copy_hugetlb_page_range > will copy the swap entry and make it COW if needed. hugetlb_fault will > back off and so the page fault is retries if the page is still under > migration and waits for its completion in hugetlb_fault. > > That means that the BUG_ON is wrong and we should update it. Let's > simply check that all present ptes are pte_huge instead. > > Reported-by: zhongjiang <zhongjiang@xxxxxxxxxx> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > --- > mm/hugetlb.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 34379d653aa3..31dd2b8b86b3 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -4303,7 +4303,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, > pte = (pte_t *)pmd_alloc(mm, pud, addr); > } > } > - BUG_ON(pte && !pte_none(*pte) && !pte_huge(*pte)); > + BUG_ON(pte && pte_present(*pte) && !pte_huge(*pte)); > > return pte; > } I don't think that the patch can fix the question. The explain is as follow. cpu0 cpu1 copy_hugetlb_page_range try_to_unmap_one huge_pte_alloc #pmd may be shared lock dst_pte #dst_pte may be migrate lock src_pte #src_pte may be normal pt1 set_huge_pte_at #dst_pte points to normal spin_unlock (src_pt1) lock src_pte spin_unlock(dst_pt1) set src_pte migrate entry spin_unlock(src_pte) * dst_pte is a normal pte, but corresponding to the pfn is under migrate. it is dangerous. The race may occur. is right ? if the scenario exist. we should think about more. Thanks zhongjiang -- 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>