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Date: Tue, 3 Sep 2019 16:28:52 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Salil Mehta <salil.mehta@huawei.com>
Subject: Re: [PATCH v2 2/9] x86: numa: check the node id consistently for x86
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On Tue, Sep 03, 2019 at 12:15:24PM +0000, Salil Mehta wrote:
> > From: Linuxarm [mailto:linuxarm-bounces@huawei.com] On Behalf Of Peter Zijlstra
> > On Tue, Sep 03, 2019 at 02:19:04PM +0800, Yunsheng Lin wrote:

> > > Is it possible that the node id set by device_add() become invalid
> > > if the node is offlined, then dev_to_node() may return a invalid
> > > node id.
> > 
> > In that case I would expect the device to go away too. Once the memory
> > controller goes away, the PCI bus connected to it cannot continue to
> > function.
> 
> I am not sure if this is *exactly* true on our system as NUMA nodes are
> part of the SoCs and devices could still be used even if all the memory
> and CPUs part of the node are turned off. Although, it is highly unlikely
> anybody would do that(maybe could be debated for the Power Management case?) 

Cute; anyway, we never change nr_node_ids (after boot), so once a node
is deemed valid it always is.

The worst that can happen in the above case, is that cpumask_of_node()
returns an empty mask, which, if all CPUs (of said node) are offline, is
an accurate representation.