Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree

[Michal Hocko <mhocko@kernel.org>]

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@dhcp22.suse.cz
> >>> ---
> >>> >From b1e9b3214f1859fdf7d134cdcb56f5871933539c Mon Sep 17 00:00:00 2001
> >>> From: Michal Hocko <mhocko@suse.com>
> >>> 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@huawei.com>
> >>> Signed-off-by: Michal Hocko <mhocko@suse.com>
> >>> ---
> >>>  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?

-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>