From: Huang Ying <ying.huang@xxxxxxxxx>
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.
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;
--
2.15.0
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