On Thu 21-07-16 18:54:09, zhong jiang wrote: > 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. Can this happen at all? copy_hugetlb_page_range does the following to rule out shared page table entries. At least that is my understanding of c5c99429fa57 ("fix hugepages leak due to pagetable page sharing") /* If the pagetables are shared don't copy or take references */ if (dst_pte == src_pte) continue; -- Michal Hocko SUSE Labs -- 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>