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[PATCH -mm -V2] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration

From: Huang Ying <ying.huang@xxxxxxxxx>

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

       /* a huge page is allocated and mapped at fault address */
       /* the huge page is under migration, leaves migration entry
          in page table */
       /* 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.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
 fs/userfaultfd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ac9a4e65ca49..cbc715fc82e1 100644
--- a/fs/userfaultfd.c
+++ b/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;