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SubjectRe: [PATCH] [4/4] SLAB: Fix node add timer race in cache_reap
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.

Slab isn't concerned about that until the memory is onlined by doing
echo online > /sys/devices/system/memory/memoryX/state for the new memory
section. This is where all the new pages are onlined, kswapd is started
on the new node, and the zonelists are built. It's also where the new
node gets set in N_HIGH_MEMORY and, thus, it's possible to call
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.

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