mm, mempool: only set __GFP_NOMEMALLOC if there are free elements

If an oom killed thread calls mempool_alloc(), it is possible that it'll
loop forever if there are no elements on the freelist since
__GFP_NOMEMALLOC prevents it from accessing needed memory reserves in
oom conditions.

Only set __GFP_NOMEMALLOC if there are elements on the freelist.  If
there are no free elements, allow allocations without the bit set so
that memory reserves can be accessed if needed.

Additionally, using mempool_alloc() with __GFP_NOMEMALLOC is not
supported since the implementation can loop forever without accessing
memory reserves when needed.
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempool.c

@@ -310,25 +310,36 @@ EXPORT_SYMBOL(mempool_resize);
  * returns NULL. Note that due to preallocation, this function
  * *never* fails when called from process contexts. (it might
  * fail if called from an IRQ context.)
- * Note: using __GFP_ZERO is not supported.
+ * Note: neither __GFP_NOMEMALLOC nor __GFP_ZERO are supported.
  */
-void * mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
+void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
 {
 	void *element;
 	unsigned long flags;
 	wait_queue_t wait;
 	gfp_t gfp_temp;
 
+	/* If oom killed, memory reserves are essential to prevent livelock */
+	VM_WARN_ON_ONCE(gfp_mask & __GFP_NOMEMALLOC);
+	/* No element size to zero on allocation */
 	VM_WARN_ON_ONCE(gfp_mask & __GFP_ZERO);
+
 	might_sleep_if(gfp_mask & __GFP_DIRECT_RECLAIM);
 
-	gfp_mask |= __GFP_NOMEMALLOC;	/* don't allocate emergency reserves */
 	gfp_mask |= __GFP_NORETRY;	/* don't loop in __alloc_pages */
 	gfp_mask |= __GFP_NOWARN;	/* failures are OK */
 
 	gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);
 
 repeat_alloc:
+	if (likely(pool->curr_nr)) {
+		/*
+		 * Don't allocate from emergency reserves if there are
+		 * elements available.  This check is racy, but it will
+		 * be rechecked each loop.
+		 */
+		gfp_temp |= __GFP_NOMEMALLOC;
+	}
 
 	element = pool->alloc(gfp_temp, pool->pool_data);
 	if (likely(element != NULL))
@@ -352,11 +363,12 @@ void * mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
 	 * We use gfp mask w/o direct reclaim or IO for the first round.  If
 	 * alloc failed with that and @pool was empty, retry immediately.
 	 */
-	if (gfp_temp != gfp_mask) {
+	if ((gfp_temp & ~__GFP_NOMEMALLOC) != gfp_mask) {
 		spin_unlock_irqrestore(&pool->lock, flags);
 		gfp_temp = gfp_mask;
 		goto repeat_alloc;
 	}
+	gfp_temp = gfp_mask;
 
 	/* We must not sleep if !__GFP_DIRECT_RECLAIM */
 	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {