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

[Michal Hocko <mhocko@suse.cz>]

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?

>  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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> 

-- 
Michal Hocko
SUSE Labs

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