mm/slab.c: fix SLAB freelist randomization duplicate entries

This patch fixes a bug in the freelist randomization code.  When a high
random number is used, the freelist will contain duplicate entries.  It
will result in different allocations sharing the same chunk.

It will result in odd behaviours and crashes.  It should be uncommon but
it depends on the machines.  We saw it happening more often on some
machines (every few hours of running tests).

Fixes: c7ce4f6 ("mm: SLAB freelist randomization")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: John Sperbeck <[email protected]>
Signed-off-by: Thomas Garnier <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: John Sperbeck

mm/slab.c

@@ -2457,7 +2457,6 @@ union freelist_init_state {
 		unsigned int pos;
 		unsigned int *list;
 		unsigned int count;
-		unsigned int rand;
 	};
 	struct rnd_state rnd_state;
 };
@@ -2483,8 +2482,7 @@ static bool freelist_state_initialize(union freelist_init_state *state,
 	} else {
 		state->list = cachep->random_seq;
 		state->count = count;
-		state->pos = 0;
-		state->rand = rand;
+		state->pos = rand % count;
 		ret = true;
 	}
 	return ret;
@@ -2493,7 +2491,9 @@ static bool freelist_state_initialize(union freelist_init_state *state,
 /* Get the next entry on the list and randomize it using a random shift */
 static freelist_idx_t next_random_slot(union freelist_init_state *state)
 {
-	return (state->list[state->pos++] + state->rand) % state->count;
+	if (state->pos >= state->count)
+		state->pos = 0;
+	return state->list[state->pos++];
 }
 
 /* Swap two freelist entries */