Re: [RFC PATCH] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables

[Mel Gorman <mgorman@suse.de>]

On Thu, Jul 19, 2012 at 04:42:13PM +0200, Michal Hocko wrote:
> [/me puts the patch destroyer glasses on]
> 

It's a super power now.

> On Wed 18-07-12 11:43:09, Mel Gorman wrote:
> [...]
> > diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> > index f6679a7..0524556 100644
> > --- a/arch/x86/mm/hugetlbpage.c
> > +++ b/arch/x86/mm/hugetlbpage.c
> > @@ -68,14 +68,37 @@ static void 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;
> > +	spinlock_t *spage_table_lock = NULL;
> > +	struct rw_semaphore *smmap_sem = NULL;
> >  
> >  	if (!vma_shareable(vma, addr))
> >  		return;
> >  
> > +retry:
> >  	mutex_lock(&mapping->i_mmap_mutex);
> >  	vma_prio_tree_foreach(svma, &iter, &mapping->i_mmap, idx, idx) {
> >  		if (svma == vma)
> >  			continue;
> > +		if (svma->vm_mm == vma->vm_mm)
> > +			continue;
> > +
> > +		/*
> > +		 * The target mm could be in the process of tearing down
> > +		 * its page tables and the i_mmap_mutex on its own is
> > +		 * not sufficient. To prevent races against teardown and
> > +		 * pagetable updates, we acquire the mmap_sem and pagetable
> > +		 * lock of the remote address space. down_read_trylock()
> > +		 * is necessary as the other process could also be trying
> > +		 * to share pagetables with the current mm.
> > +		 */
> > +		if (!down_read_trylock(&svma->vm_mm->mmap_sem)) {
> > +			mutex_unlock(&mapping->i_mmap_mutex);
> > +			goto retry;
> > +		}
> > +
> 
> I am afraid this can easily cause a dead lock. Consider
> fork
>   dup_mmap
>     down_write(&oldmm->mmap_sem)
>     copy_page_range
>       copy_hugetlb_page_range
>         huge_pte_alloc
> 
> svma could belong to oldmm and then we would loop for ever. 
> svma->vm_mm == vma->vm_mm doesn't help because vma is child's one and mm
> differ in that case. I am wondering you didn't hit this while testing.
> It would suggest that the ptes are not populated yet because we didn't
> let parent play and then other children could place its vma in the list
> before parent?
> 

Yes, I think you're right - both about the race and why I didn't hit it.
The libhugetlbfs tests probably avoided the bug for the same reason.
Thanks for pointing this out.

-- 
Mel Gorman
SUSE Labs

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