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

[zhong jiang <zhongjiang@huawei.com>]

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.

Thanks
zhongjiang


       
      
 
        
          
 

--
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>