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SubjectRe: [patch] mm, slab: suppress out of memory warning unless debug is enabled
On Wed, 7 May 2014, Andrew Morton 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.
>

You think the spam in http://marc.info/?l=linux-kernel&m=139927773010514
is meaningful? It also looks like two different errors when in reality it
is a single allocation.

Unless you're debugging a slab issue, all the pertinent information is
already available in the page allocation failure warning emitted by the
page allocator: we already have the order and gfp mask. We also know it's
a slab allocation because of the __kmalloc in the call trace.

Does this user care about that there are 207 slabs on node 0 with 207
objects? Probably only if they are diagnosing a slab problem.

> > @@ -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.
>

Agreed, but it is a testiment to the uselessness of this information
already. The page allocation failure warnings are controlled by their own
ratelimiter, nopage_rs, but that's local to the page allocator. Do you
prefer that all these ratelimiters be moved to the global namespace for
generic use?


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