ext4: limit group search loop for non-extent files

Orabug: 17488415

commit e615573 upstream.

In the case where we are allocating for a non-extent file,
we must limit the groups we allocate from to those below
2^32 blocks, and ext4_mb_regular_allocator() attempts to
do this initially by putting a cap on ngroups for the
subsequent search loop.

However, the initial target group comes in from the
allocation context (ac), and it may already be beyond
the artificially limited ngroups.  In this case,
the limit

	if (group == ngroups)
		group = 0;

at the top of the loop is never true, and the loop will
run away.

Catch this case inside the loop and reset the search to
start at group 0.

[[email protected]: add commit msg & comments]

Signed-off-by: Lachlan McIlroy <[email protected]>
Signed-off-by: Eric Sandeen <[email protected]>
Signed-off-by: "Theodore Ts'o" <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

(cherry picked from commit 7fb7465071b6f553c5c5cd8aca704cfc6896917c)

Signed-off-by: Todd Vierling <[email protected]>
Acked-by: John Haxby <[email protected]>
Signed-off-by: Brian Maly <[email protected]>

fs/ext4/mballoc.c

@@ -2026,7 +2026,11 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		group = ac->ac_g_ex.fe_group;
 
 		for (i = 0; i < ngroups; group++, i++) {
-			if (group == ngroups)
+			/*
+			 * Artificially restricted ngroups for non-extent
+			 * files makes group > ngroups possible on first loop.
+			 */
+			if (group >= ngroups)
 				group = 0;
 
 			/* This now checks without needing the buddy page */