mm, userfaultfd, THP: avoid waiting when PMD under THP migration

If THP migration is enabled, for a VMA handled by userfaultfd, consider
the following situation,

  do_page_fault()
    __do_huge_pmd_anonymous_page()
     handle_userfault()
       userfault_msg()
         /* a huge page is allocated and mapped at fault address */
         /* the huge page is under migration, leaves migration entry
            in page table */
       userfaultfd_must_wait()
         /* return true because !pmd_present() */
       /* may wait in loop until fatal signal */

That is, it may be possible for userfaultfd_must_wait() encounters a PMD
entry which is !pmd_none() && !pmd_present().  In the current
implementation, we will wait for such PMD entries, which may cause
unnecessary waiting, and potential soft lockup.

This is fixed via avoiding to wait when !pmd_none() && !pmd_present(),
only wait when pmd_none().

This may be not a problem in practice, because userfaultfd_must_wait()
is always called with mm->mmap_sem read-locked.  mremap() will
write-lock mm->mmap_sem.  And UFFDIO_COPY doesn't support to copy THP
mapping.  But the change introduced still makes the code more correct,
and makes the PMD and PTE code more consistent.

Link: http://lkml.kernel.org/r/20171207011752.3292-1-ying.huang@intel.comSigned-off-by: N"Huang, Ying" <ying.huang@intel.com>
Reviewed-by: NAndrea Arcangeli <aarcange@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.UK>
Cc: Zi Yan <zi.yan@cs.rutgers.edu>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -294,10 +294,13 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 	 * pmd_trans_unstable) of the pmd.
 	 */
 	_pmd = READ_ONCE(*pmd);
-	if (!pmd_present(_pmd))
+	if (pmd_none(_pmd))
 		goto out;
 
 	ret = false;
+	if (!pmd_present(_pmd))
+		goto out;
+
 	if (pmd_trans_huge(_pmd))
 		goto out;