mm: introduce FAULT_FLAG_DEFAULT

Although there're tons of arch-specific page fault handlers, most of them
are still sharing the same initial value of the page fault flags.  Say,
merely all of the page fault handlers would allow the fault to be retried,
and they also allow the fault to respond to SIGKILL.

Let's define a default value for the fault flags to replace those initial
page fault flags that were copied over.  With this, it'll be far easier to
introduce new fault flag that can be used by all the architectures instead
of touching all the archs.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Brian Geffon <bgeffon@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Martin Cracauer <cracauer@cons.org>
Cc: Marty McFadden <mcfadden8@llnl.gov>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Maya Gokhale <gokhale2@llnl.gov>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Link: http://lkml.kernel.org/r/20200220160238.9694-1-peterx@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -391,6 +391,13 @@ extern pgprot_t protection_map[16];
 #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
 #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
 
+/*
+ * The default fault flags that should be used by most of the
+ * arch-specific page fault handlers.
+ */
+#define FAULT_FLAG_DEFAULT  (FAULT_FLAG_ALLOW_RETRY | \
+			     FAULT_FLAG_KILLABLE)
+
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
 	{ FAULT_FLAG_MKWRITE,		"MKWRITE" }, \