The patch titled
Subject: mm, userfaultfd, THP: avoid waiting when PMD under THP migration
has been added to the -mm tree. Its filename is
mm-userfaultfd-thp-avoid-waiting-when-pmd-under-thp-migration.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-userfaultfd-thp-avoid-waiting-when-pmd-under-thp-migration.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-userfaultfd-thp-avoid-waiting-when-pmd-under-thp-migration.patch
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------------------------------------------------------
From: Huang Ying <ying.huang@xxxxxxxxx>
Subject: 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@xxxxxxxxx
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>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/userfaultfd.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff -puN fs/userfaultfd.c~mm-userfaultfd-thp-avoid-waiting-when-pmd-under-thp-migration fs/userfaultfd.c
--- a/fs/userfaultfd.c~mm-userfaultfd-thp-avoid-waiting-when-pmd-under-thp-migration
+++ a/fs/userfaultfd.c
@@ -294,10 +294,13 @@ static inline bool userfaultfd_must_wait
* 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;
_
Patches currently in -mm which might be from ying.huang@xxxxxxxxx are
mm-userfaultfd-thp-avoid-waiting-when-pmd-under-thp-migration.patch
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