Re: [PATCH] mm/hugetlb: fix race when migrate pages.

[zhong jiang <zhongjiang@huawei.com>]

On 2016/7/19 19:10, Michal Hocko wrote:
> On Tue 19-07-16 17:52:50, zhong jiang wrote:
>> On 2016/7/19 17:17, Michal Hocko wrote:
> [...]
>>> OK, so this states the problem. Although it would be helpful to be
>>> specific about which BUG has triggered because the above line doesn't
>>> match any in the current code. I assume this is 
>>>
>>> BUG_ON(pte && !pte_none(*pte) && !pte_huge(*pte))
>>>
>>> in huge_pte_alloc. Now the changelog is silent about what the actual
>>> problem is and what is the fix. Could you add this information please?
>>   Yes, it hit this BUG_ON() you had mentioned.  The pmd share function enable,  when
>>   I run online-offline memory , That will lead to pte_huge() return false.  beacuse
>>  it refer to the pmd may be ongoing  migration.
> OK, I see. But is the proposed fix correct? AFAIU you are retrying the
> VMA walk and nothing really prevents huge_pte_offset returning the same
> spte, right?
>  
   oh, I mistaked.  we should not repeat, it should directly  skip to end. 
  diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6384dfd..baba196 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4213,7 +4213,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
        struct vm_area_struct *svma;
        unsigned long saddr;
        pte_t *spte = NULL;
-       pte_t *pte;
+       pte_t *pte, entry;
        spinlock_t *ptl;

        if (!vma_shareable(vma, addr))
@@ -4240,6 +4240,11 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)

        ptl = huge_pte_lockptr(hstate_vma(vma), mm, spte);
        spin_lock(ptl);
+       entry = huge_ptep_get(spte);
+       if (is_hugetlb_entry_migration(entry) ||
+                       is_hugetlb_entry_hwpoisoned(entry)) {
+               goto end;
+       }
        if (pud_none(*pud)) {
                pud_populate(mm, pud,
                                (pmd_t *)((unsigned long)spte & PAGE_MASK));
@@ -4247,6 +4252,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
                put_page(virt_to_page(spte));
                mm_dec_nr_pmds(mm);
        }
+end:
        spin_unlock(ptl);
 out:
        pte = (pte_t *)pmd_alloc(mm, pud, addr);

>>  Thanks
>>  zhong jiang
>>>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
>>>> ---
>>>>  mm/hugetlb.c | 9 ++++++++-
>>>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index 6384dfd..1b54d7a 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -4213,13 +4213,14 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
>>>>  	struct vm_area_struct *svma;
>>>>  	unsigned long saddr;
>>>>  	pte_t *spte = NULL;
>>>> -	pte_t *pte;
>>>> +	pte_t *pte, entry;
>>>>  	spinlock_t *ptl;
>>>>  
>>>>  	if (!vma_shareable(vma, addr))
>>>>  		return (pte_t *)pmd_alloc(mm, pud, addr);
>>>>  
>>>>  	i_mmap_lock_write(mapping);
>>>> +retry:
>>>>  	vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
>>>>  		if (svma == vma)
>>>>  			continue;
>>>> @@ -4240,6 +4241,12 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
>>>>  
>>>>  	ptl = huge_pte_lockptr(hstate_vma(vma), mm, spte);
>>>>  	spin_lock(ptl);
>>>> +	entry = huge_ptep_get(spte);
>>>> + 	if (is_hugetlb_entry_migration(entry) || 
>>>> +			is_hugetlb_entry_hwpoisoned(entry)) {
>>>> +		spin_unlock(ptl);
>>>> +		goto retry;
>>>> +	}	
>>>>  	if (pud_none(*pud)) {
>>>>  		pud_populate(mm, pud,
>>>>  				(pmd_t *)((unsigned long)spte & PAGE_MASK));
>>>> -- 
>>>> 1.8.3.1
>>>>
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