linux-kernel - Re: [PATCH] [4/4] SLAB: Fix node add timer race in cache

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Date:	Thu, 25 Feb 2010 16:31:00 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Andi Kleen <andi@...stfloor.org>,
	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, haicheng.li@...el.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] [4/4] SLAB: Fix node add timer race in cache_reap

On Thu, 25 Feb 2010, David Rientjes wrote:

> On Thu, 25 Feb 2010, Christoph Lameter wrote:
>
> > > I don't see how memory hotadd with a new node being onlined could have
> > > worked fine before since slab lacked any memory hotplug notifier until
> > > Andi just added it.
> >
> > AFAICR The cpu notifier took on that role in the past.
> >
>
> The cpu notifier isn't involved if the firmware notifies the kernel that a
> new ACPI memory device has been added or you write a start address to
> /sys/devices/system/memory/probe.  Hot-added memory devices can include
> ACPI_SRAT_MEM_HOT_PLUGGABLE entries in the SRAT for x86 that assign them
> non-online node ids (although all such entries get their bits set in
> node_possible_map at boot), so a new pgdat may be allocated for the node's
> registered range.

Yes Andi's work makes it explicit but there is already code in the cpu
notifier (see cpuup_prepare) that seems to have been intended to
initialize the node structures. Wonder why the hotplug people never
addressed that issue? Kame?


      list_for_each_entry(cachep, &cache_chain, next) {
                /*
                 * Set up the size64 kmemlist for cpu before we can
                 * begin anything. Make sure some other cpu on this
                 * node has not already allocated this
                 */
                if (!cachep->nodelists[node]) {
                        l3 = kmalloc_node(memsize, GFP_KERNEL, node);
                        if (!l3)
                                goto bad;
                        kmem_list3_init(l3);
                        l3->next_reap = jiffies + REAPTIMEOUT_LIST3 +
                            ((unsigned long)cachep) % REAPTIMEOUT_LIST3;

                        /*
                         * The l3s don't come and go as CPUs come and
                         * go.  cache_chain_mutex is sufficient
                         * protection here.
                         */
                        cachep->nodelists[node] = l3;
                }

                spin_lock_irq(&cachep->nodelists[node]->list_lock);
                cachep->nodelists[node]->free_limit =
                        (1 + nr_cpus_node(node)) *
                        cachep->batchcount + cachep->num;
                spin_unlock_irq(&cachep->nodelists[node]->list_lock);
        }


> kmalloc_node() in generic kernel code.  All that is done under
> MEM_GOING_ONLINE and not MEM_ONLINE, which is why I suggest the first and
> fourth patch in this series may not be necessary if we prevent setting the
> bit in the nodemask or building the zonelists until the slab nodelists are
> ready.

That sounds good.


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