Re: [MM Bug?] mmap() triggers SIGBUS while doing the​ ​numa_move_pages() for offlined hugepage in background

[Michal Hocko <mhocko@suse.com>]

On Fri 02-08-19 10:42:33, Mike Kravetz wrote:
> On 8/1/19 9:15 PM, Naoya Horiguchi wrote:
> > On Thu, Aug 01, 2019 at 05:19:41PM -0700, Mike Kravetz wrote:
> >> There appears to be a race with hugetlb_fault and try_to_unmap_one of
> >> the migration path.
> >>
> >> Can you try this patch in your environment?  I am not sure if it will
> >> be the final fix, but just wanted to see if it addresses issue for you.
> >>
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index ede7e7f5d1ab..f3156c5432e3 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -3856,6 +3856,20 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
> >>  
> >>  		page = alloc_huge_page(vma, haddr, 0);
> >>  		if (IS_ERR(page)) {
> >> +			/*
> >> +			 * We could race with page migration (try_to_unmap_one)
> >> +			 * which is modifying page table with lock.  However,
> >> +			 * we are not holding lock here.  Before returning
> >> +			 * error that will SIGBUS caller, get ptl and make
> >> +			 * sure there really is no entry.
> >> +			 */
> >> +			ptl = huge_pte_lock(h, mm, ptep);
> >> +			if (!huge_pte_none(huge_ptep_get(ptep))) {
> >> +				ret = 0;
> >> +				spin_unlock(ptl);
> >> +				goto out;
> >> +			}
> >> +			spin_unlock(ptl);
> > 
> > Thanks you for investigation, Mike.
> > I tried this change and found no SIGBUS, so it works well.
> > 
> > I'm still not clear about how !huge_pte_none() becomes true here,
> > because we enter hugetlb_no_page() only when huge_pte_none() is non-null
> > and (racy) try_to_unmap_one() from page migration should convert the
> > huge_pte into a migration entry, not null.
> 
> Thanks for taking a look Naoya.
> 
> In try_to_unmap_one(), there is this code block:
> 
> 		/* Nuke the page table entry. */
> 		flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
> 		if (should_defer_flush(mm, flags)) {
> 			/*
> 			 * We clear the PTE but do not flush so potentially
> 			 * a remote CPU could still be writing to the page.
> 			 * If the entry was previously clean then the
> 			 * architecture must guarantee that a clear->dirty
> 			 * transition on a cached TLB entry is written through
> 			 * and traps if the PTE is unmapped.
> 			 */
> 			pteval = ptep_get_and_clear(mm, address, pvmw.pte);
> 
> 			set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
> 		} else {
> 			pteval = ptep_clear_flush(vma, address, pvmw.pte);
> 		}
> 
> That happens before setting the migration entry.  Therefore, for a period
> of time the pte is NULL (huge_pte_none() returns true).
> 
> try_to_unmap_one holds the page table lock, but hugetlb_fault does not take
> the lock to 'optimistically' check huge_pte_none().  When huge_pte_none
> returns true, it calls hugetlb_no_page which is where we try to allocate
> a page and fails.
> 
> Does that make sense, or am I missing something?
> 
> The patch checks for this specific condition: someone changing the pte
> from NULL to non-NULL while holding the lock.  I am not sure if this is
> the best way to fix.  But, it may be the easiest.

Please add a comment to explain this because this is quite subtle and
tricky. Unlike the regular page fault hugetlb_no_page is protected by a
large lock so a retry check seems unexpected.

Thanks!
-- 
Michal Hocko
SUSE Labs