Re: [PATCH v5 2/4] cacheinfo: Allocate memory for memory if not done from the primary CPU

[Nikolay Borisov <nik.borisov@suse.com>]

On 27.08.24 г. 8:16 ч., 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.

I assume instead of duplicating "memory" in the subject you meant 
"cacheinfo" ?

Otherwise LGTM:

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>

> 
> Reviewed-by: Radu Rendec <rrendec@redhat.com>
> Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
> Fixes: 6539cffa9495 ("cacheinfo: Add arch specific early level initializer")
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Cc: Andreas Herrmann <aherrmann@suse.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Chen Yu <yu.c.chen@intel.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Len Brown <len.brown@intel.com>
> Cc: Nikolay Borisov <nik.borisov@suse.com>
> Cc: Radu Rendec <rrendec@redhat.com>
> Cc: Pierre Gondois <Pierre.Gondois@arm.com>
> Cc: Pu Wen <puwen@hygon.cn>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
> Cc: Sudeep Holla <sudeep.holla@arm.com>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Zhang Rui <rui.zhang@intel.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: stable@vger.kernel.org # 6.3+
> ---
> The motivation for commit 5944ce092b97 was to prevent a BUG splat in
> PREEMPT_RT kernels during memory allocation. This splat is not observed on
> x86 because the memory allocation for cacheinfo happens in
> detect_cache_attributes() from the cacheinfo CPU hotplug callback.
> 
> The dereference of a NULL pointer is not observed today because
> cache_leaves(cpu) is zero until after init_cache_level() is called (also
> during the CPU hotplug callback). A subsequent changeset will set the
> number of cache leaves earlier and the NULL-pointer dereference will be
> observed.
> ---
> Changes since v4:
>   * None
> 
> Changes since v3:
>   * Added Reviewed-by tag from Radu and Sudeep. Thanks!
> 
> Changes since v2:
>   * Introduced this patch.
> 
> Changes since v1:
>   * N/A
> ---
>   drivers/base/cacheinfo.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> index 77f2e0f91589..0332148691f9 100644
> --- a/drivers/base/cacheinfo.c
> +++ b/drivers/base/cacheinfo.c
> @@ -554,7 +554,11 @@ static inline int init_level_allocate_ci(unsigned int cpu)
>   	 */
>   	ci_cacheinfo(cpu)->early_ci_levels = false;
>   
> -	if (cache_leaves(cpu) <= early_leaves)
> +	/*
> +	 * Some architectures (e.g., x86) do not use early initialization.
> +	 * Allocate memory now in such case.
> +	 */
> +	if (cache_leaves(cpu) <= early_leaves && per_cpu_cacheinfo(cpu))
>   		return 0;
>   
>   	kfree(per_cpu_cacheinfo(cpu));