mm: introduce memalloc_noreclaim_{save,restore}

The previous patch ("mm: prevent potential recursive reclaim due to
clearing PF_MEMALLOC") has shown that simply setting and clearing
PF_MEMALLOC in current->flags can result in wrongly clearing a
pre-existing PF_MEMALLOC flag and potentially lead to recursive reclaim.
Let's introduce helpers that support proper nesting by saving the
previous stat of the flag, similar to the existing memalloc_noio_* and
memalloc_nofs_* helpers.  Convert existing setting/clearing of
PF_MEMALLOC within mm to the new helpers.

There are no known issues with the converted code, but the change makes
it more robust.

Link: http://lkml.kernel.org/r/20170405074700.29871-3-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Chris Leech <cleech@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Lee Duncan <lduncan@suse.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 9daabe138c99..2b24a6974847 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -191,4 +191,16 @@ static inline void memalloc_nofs_restore(unsigned int flags)
 	current->flags = (current->flags & ~PF_MEMALLOC_NOFS) | flags;
 }
 
+static inline unsigned int memalloc_noreclaim_save(void)
+{
+	unsigned int flags = current->flags & PF_MEMALLOC;
+	current->flags |= PF_MEMALLOC;
+	return flags;
+}
+
+static inline void memalloc_noreclaim_restore(unsigned int flags)
+{
+	current->flags = (current->flags & ~PF_MEMALLOC) | flags;
+}
+
 #endif /* _LINUX_SCHED_MM_H */