linux-kernel - Re: [PATCH v7] numa: make node_to_cpumask_map() NUMA_NO

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Date:   Wed, 30 Oct 2019 11:22:29 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>, catalin.marinas@....com,
        will@...nel.org, mingo@...hat.com, bp@...en8.de, rth@...ddle.net,
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Subject: Re: [PATCH v7] numa: make node_to_cpumask_map() NUMA_NO_NODE aware

On Wed 30-10-19 11:14:49, Peter Zijlstra wrote:
> On Wed, Oct 30, 2019 at 05:34:28PM +0800, Yunsheng Lin wrote:
> > When passing the return value of dev_to_node() to cpumask_of_node()
> > without checking if the device's node id is NUMA_NO_NODE, there is
> > global-out-of-bounds detected by KASAN.
> > 
> > From the discussion [1], NUMA_NO_NODE really means no node affinity,
> > which also means all cpus should be usable. So the cpumask_of_node()
> > should always return all cpus online when user passes the node id as
> > NUMA_NO_NODE, just like similar semantic that page allocator handles
> > NUMA_NO_NODE.
> > 
> > But we cannot really copy the page allocator logic. Simply because the
> > page allocator doesn't enforce the near node affinity. It just picks it
> > up as a preferred node but then it is free to fallback to any other numa
> > node. This is not the case here and node_to_cpumask_map will only restrict
> > to the particular node's cpus which would have really non deterministic
> > behavior depending on where the code is executed. So in fact we really
> > want to return cpu_online_mask for NUMA_NO_NODE.
> > 
> > Also there is a debugging version of node_to_cpumask_map() for x86 and
> > arm64, which is only used when CONFIG_DEBUG_PER_CPU_MAPS is defined, this
> > patch changes it to handle NUMA_NO_NODE as normal node_to_cpumask_map().
> > 
> > [1] https://lkml.org/lkml/2019/9/11/66
> > Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> > Suggested-by: Michal Hocko <mhocko@...nel.org>
> > Acked-by: Michal Hocko <mhocko@...e.com>
> > Acked-by: Paul Burton <paul.burton@...s.com> # MIPS bits
> 
> Still:
> 
> Nacked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Do you have any other proposal that doesn't make any wild guesses about
which node to use instead of the undefined one?

-- 
Michal Hocko
SUSE Labs