logfs: fix deadlock in logfs_get_wblocks, hold and wait on super->s_write_mutex

do_logfs_journal_wl_pass() should use GFP_NOFS for memory allocation GC
code calls btree_insert32 with GFP_KERNEL while holding a mutex
super->s_write_mutex.

The same mutex is used in address_space_operations->writepage(), and a
call to writepage() could be triggered as a result of memory allocation
in btree_insert32, causing a deadlock.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=20342

Signed-off-by: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Joern Engel <joern@logfs.org>
Cc: Florian Mickler <florian@mickler.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Maciej Rutecki <maciej.rutecki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/logfs/journal.c

@@ -828,7 +828,7 @@ void do_logfs_journal_wl_pass(struct super_block *sb)
 		super->s_journal_seg[i] = segno;
 		super->s_journal_ec[i] = ec;
 		logfs_set_segment_reserved(sb, segno);
-		err = btree_insert32(head, segno, (void *)1, GFP_KERNEL);
+		err = btree_insert32(head, segno, (void *)1, GFP_NOFS);
 		BUG_ON(err); /* mempool should prevent this */
 		err = logfs_erase_segment(sb, segno, 1);
 		BUG_ON(err); /* FIXME: remount-ro would be nicer */