On 2016/7/21 19:27, Michal Hocko wrote: > 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; vm_file points to mapping should be shared, I am not sure, if it is so, the possibility is exist. of course, src_pte is the same as the dst_pte. when dst_pte is migrate entry and src pte is normal entry, if try_to_unmap_one is successful, then exec copy_hugetlb_page_range, it will lead to the dst_pte is under dangerous. -- 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>