linux-kernel - [PATCH v5 09/16] mm: Return faster for non-fatal signals in user mode faults

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Message-Id: <20190926093904.5090-10-peterx@redhat.com>
Date:   Thu, 26 Sep 2019 17:38:57 +0800
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Jerome Glisse <jglisse@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>, peterx@...hat.com,
        Martin Cracauer <cracauer@...s.org>,
        Matthew Wilcox <willy@...radead.org>, Shaohua Li <shli@...com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: [PATCH v5 09/16] mm: Return faster for non-fatal signals in user mode faults

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@...ux-foundation.org>
Suggested-by: Andrea Arcangeli <aarcange@...hat.com>
Signed-off-by: Peter Xu <peterx@...hat.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 46429192733b..031af0a6505a 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -387,7 +387,8 @@ 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))));
 }
 
 /*
-- 
2.21.0