The patch titled
Subject: mm/gup.c: teach get_user_pages_unlocked to handle FOLL_NOWAIT
has been added to the -mm tree. Its filename is
mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait.patch
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------------------------------------------------------
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Subject: mm/gup.c: teach get_user_pages_unlocked to handle FOLL_NOWAIT
KVM is hanging during postcopy live migration with userfaultfd because
get_user_pages_unlocked is not capable to handle FOLL_NOWAIT.
Earlier FOLL_NOWAIT was only ever passed to get_user_pages.
Specifically faultin_page (the callee of get_user_pages_unlocked caller)
doesn't know that if FAULT_FLAG_RETRY_NOWAIT was set in the page fault
flags, when VM_FAULT_RETRY is returned, the mmap_sem wasn't actually
released (even if nonblocking is not NULL). So it sets *nonblocking to
zero and the caller won't release the mmap_sem thinking it was already
released, but it wasn't because of FOLL_NOWAIT.
Link: http://lkml.kernel.org/r/20180302174343.5421-2-aarcange@xxxxxxxxxx
Fixes: ce53053ce378c ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()")
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Reported-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
Tested-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/gup.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff -puN mm/gup.c~mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait mm/gup.c
--- a/mm/gup.c~mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait
+++ a/mm/gup.c
@@ -516,7 +516,7 @@ static int faultin_page(struct task_stru
}
if (ret & VM_FAULT_RETRY) {
- if (nonblocking)
+ if (nonblocking && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
*nonblocking = 0;
return -EBUSY;
}
@@ -890,7 +890,10 @@ static __always_inline long __get_user_p
break;
}
if (*locked) {
- /* VM_FAULT_RETRY didn't trigger */
+ /*
+ * VM_FAULT_RETRY didn't trigger or it was a
+ * FOLL_NOWAIT.
+ */
if (!pages_done)
pages_done = ret;
break;
_
Patches currently in -mm which might be from aarcange@xxxxxxxxxx are
mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait.patch