Re: [PATCH v6 2/4] cacheinfo: Allocate memory during CPU hotplug if not done from the primary CPU

From: Borislav Petkov
Date: Wed Sep 11 2024 - 10:09:30 EST


On Wed, Sep 04, 2024 at 11:00:34PM -0700, Ricardo Neri wrote:
> Commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU")
> adds functionality that architectures can use to optionally allocate and
> build cacheinfo early during boot. Commit 6539cffa9495 ("cacheinfo: Add
> arch specific early level initializer") lets secondary CPUs correct (and
> reallocate memory) cacheinfo data if needed.
>
> If the early build functionality is not used and cacheinfo does not need
> correction, memory for cacheinfo is never allocated. x86 does not use the
> early build functionality. Consequently, during the cacheinfo CPU hotplug
> callback, last_level_cache_is_valid() attempts to dereference a NULL
> pointer:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000100
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not present page
> PGD 0 P4D 0
> Oops: 0000 [#1] PREEPMT SMP NOPTI
> CPU: 0 PID 19 Comm: cpuhp/0 Not tainted 6.4.0-rc2 #1
> RIP: 0010: last_level_cache_is_valid+0x95/0xe0a
>
> Allocate memory for cacheinfo during the cacheinfo CPU hotplug callback if
> not done earlier.

Why is this a separate patch?

It sounds like it should be merged with the first one as both address a CPU
hotplug issue AFAICT.

--
Regards/Gruss,
Boris.

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