lkml.org 
[lkml]   [2017]   [Nov]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration
Date
From: Huang Ying <ying.huang@intel.com>

If THP migration is enabled, the following situation is possible,

- A THP is mapped at source address
- Migration is started to move the THP to another node
- Page fault occurs
- The PMD (migration entry) is copied to the destination address in mremap

That is, it is possible for handle_userfault() encounter a PMD entry
which has been handled but !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_present(), only wait when
pmd_none().

Question:

I found userfaultfd_must_wait() is always called when PMD or PTE is
none, and with mm->mmap_sem read-lock held. mremap() will write-lock
mm->mmap_sem. And UFFDIO_COPY don't support to copy THP mapping. So
the situation described above couldn't happen in practice?

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: 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>
---
fs/userfaultfd.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index b5a0193e1960..0fcf66c3e439 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;

--
2.14.2
\
 
 \ /
  Last update: 2017-11-03 08:54    [W:1.242 / U:0.328 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site