linux-kernel - Re: [patch] mm, slab: suppress out of memory warning unless debug is enabled

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Date:	Wed, 7 May 2014 14:29:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, slab: suppress out of memory warning unless debug
 is enabled

On Wed, 7 May 2014 14:19:19 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:

> When the slab or slub allocators cannot allocate additional slab pages, they 
> emit diagnostic information to the kernel log such as current number of slabs, 
> number of objects, active objects, etc.  This is always coupled with a page 
> allocation failure warning since it is controlled by !__GFP_NOWARN.
> 
> Suppress this out of memory warning if the allocator is configured without debug 
> supported.  The page allocation failure warning will indicate it is a failed 
> slab allocation, so this is only useful to diagnose allocator bugs.
> 
> Since CONFIG_SLUB_DEBUG is already enabled by default for the slub allocator, 
> there is no functional change with this patch.  If debug is disabled, however, 
> the warnings are now suppressed.
> 

I'm not seeing any reason for making this change.

> @@ -1621,11 +1621,17 @@ __initcall(cpucache_init);
>  static noinline void
>  slab_out_of_memory(struct kmem_cache *cachep, gfp_t gfpflags, int nodeid)
>  {
> +#if DEBUG
>  	struct kmem_cache_node *n;
>  	struct page *page;
>  	unsigned long flags;
>  	int node;
>  
> +	if (gfpflags & __GFP_NOWARN)
> +		return;
> +	if (!printk_ratelimit())
> +		return;

printk_ratelimit() is lame - it uses a single global state.  So if
random net driver is using printk_ratelimit(), that driver and slab
will interfere with each other.

We don't appear to presently have a handy macro to do this properly -
you might care to add one and switch printk_ratelimited() and
pr_debug_ratelimited() over to using it.  And various sites in
include/linux/device.h, I guess.


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