jbd2: make the whole kjournald2 kthread NOFS safe

kjournald2 is central to the transaction commit processing.  As such any
potential allocation from this kernel thread has to be GFP_NOFS.  Make
sure to mark the whole kernel thread GFP_NOFS by the memalloc_nofs_save.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/20170306131408.9828-8-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Chris Mason <clm@fb.com>
Cc: David Sterba <dsterba@suse.cz>
Cc: Brian Foster <bfoster@redhat.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Nikolay Borisov <nborisov@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/jbd2/journal.c

@@ -43,6 +43,7 @@
 #include <linux/backing-dev.h>
 #include <linux/bitops.h>
 #include <linux/ratelimit.h>
+#include <linux/sched/mm.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/jbd2.h>
@@ -205,6 +206,14 @@ static int kjournald2(void *arg)
 	journal->j_task = current;
 	wake_up(&journal->j_wait_done_commit);
 
+	/*
+	 * Make sure that no allocations from this kernel thread will ever
+	 * recurse to the fs layer because we are responsible for the
+	 * transaction commit and any fs involvement might get stuck waiting for
+	 * the trasn. commit.
+	 */
+	memalloc_nofs_save();
+
 	/*
 	 * And now, wait forever for commit wakeup events.
 	 */