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Date: Wed, 16 Apr 2014 14:15:56 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Sebastian Ott <sebott@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/mempool: warn about __GFP_ZERO usage
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On Mon, 14 Apr 2014 19:23:34 +0200 (CEST) Sebastian Ott <sebott@linux.vnet.ibm.com> wrote:

> Memory obtained via mempool_alloc is not always zeroed even when
> called with __GFP_ZERO. Add a note and VM_BUG_ON statement to make
> that clear.
> 
> ..
>
> @@ -200,6 +201,7 @@ void * mempool_alloc(mempool_t *pool, gf
>  	wait_queue_t wait;
>  	gfp_t gfp_temp;
>  
> +	VM_BUG_ON(gfp_mask & __GFP_ZERO);
>  	might_sleep_if(gfp_mask & __GFP_WAIT);
>  
>  	gfp_mask |= __GFP_NOMEMALLOC;	/* don't allocate emergency reserves */

hm, BUG is a bit harsh.  How about

--- a/mm/mempool.c~mm-mempool-warn-about-__gfp_zero-usage-fix
+++ a/mm/mempool.c
@@ -201,7 +201,9 @@ void * mempool_alloc(mempool_t *pool, gf
 	wait_queue_t wait;
 	gfp_t gfp_temp;
 
-	VM_BUG_ON(gfp_mask & __GFP_ZERO);
+#ifdef CONFIG_DEBUG_VM
+	WARN_ON_ONCE(gfp_mask & __GFP_ZERO);	/* Where's VM_WARN_ON_ONCE()? */
+#endif
 	might_sleep_if(gfp_mask & __GFP_WAIT);
 
 	gfp_mask |= __GFP_NOMEMALLOC;	/* don't allocate emergency reserves */

?