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.

Link: http://lkml.kernel.org/r/20160721074340.GA26398@dhcp22.suse.cz
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: zhongjiang <zhongjiang@huawei.com>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 619e00d..ef96830 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4310,7 +4310,7 @@ 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;
 }