[PATCH RESEND v6 09/16] mm: Return faster for non-fatal signals in user mode faults

[Peter Xu <peterx@redhat.com>]

The idea comes from the upstream discussion between Linus and Andrea:

  https://lore.kernel.org/lkml/20171102193644.GB22686@redhat.com/

A summary to the issue: there was a special path in handle_userfault()
in the past that we'll return a VM_FAULT_NOPAGE when we detected
non-fatal signals when waiting for userfault handling.  We did that by
reacquiring the mmap_sem before returning.  However that brings a risk
in that the vmas might have changed when we retake the mmap_sem and
even we could be holding an invalid vma structure.

This patch is a preparation of removing that special path by allowing
the page fault to return even faster if we were interrupted by a
non-fatal signal during a user-mode page fault handling routine.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/sched/signal.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 4c87ffce64d1..09d40ce6a162 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -379,7 +379,8 @@ static inline bool fault_signal_pending(unsigned int fault_flags,
 					struct pt_regs *regs)
 {
 	return unlikely((fault_flags & VM_FAULT_RETRY) &&
-			fatal_signal_pending(current));
+			(fatal_signal_pending(current) ||
+			 (user_mode(regs) && signal_pending(current))));
 }
 
 /*
-- 
2.24.1