diff options
| author | Huang Ying <ying.huang@intel.com> | 2018-01-31 16:17:32 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2018-01-31 17:18:37 -0800 |
| commit | a365ac09d334389bc69841c9d153f03fa2442f1c (patch) | |
| tree | 54c35ebe10a2de98640f59723c3a2f1db9af1bfd /fs | |
| parent | 9bebc09fcf4fb25e36cf86af764c038b92f64057 (diff) | |
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.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Andrea 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/userfaultfd.c | 5 |
1 files changed, 4 insertions, 1 deletions
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 743eaa646898..a9d0ddc12ace 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; |