On 2016/7/21 20:30, Michal Hocko wrote: > On Thu 21-07-16 20:14:41, zhong jiang wrote: >> 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. > I am not sure I understand. This is a fork path where the ptes are > copied over from the parent to the child. So how would vm_file differ? I think you can misunderstand my meaning. A file refers to the mapping field can be shared by other process, parent process have the mapping , but is not only. This is only my viewpoint. is right ?? -- 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>