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authorHuang Ying <ying.huang@intel.com>2018-01-31 16:17:32 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2018-01-31 17:18:37 -0800
commita365ac09d334389bc69841c9d153f03fa2442f1c (patch)
tree54c35ebe10a2de98640f59723c3a2f1db9af1bfd /fs/userfaultfd.c
parent9bebc09fcf4fb25e36cf86af764c038b92f64057 (diff)
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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/userfaultfd.c')
-rw-r--r--fs/userfaultfd.c5
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;