Re: [PATCH v8 1/2] cacheinfo: Allocate memory during CPU hotplug if not done from the primary CPU

[Ricardo Neri]

On Mon, Dec 02, 2024 at 09:02:03AM +0100, Andreas Herrmann wrote:
> On Wed, Nov 27, 2024 at 04:22:46PM -0800, 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.
> > 
> > Moreover, before determining the validity of the last-level cache info,
> > ensure that it has been allocated. Simply checking for non-zero
> > cache_leaves() is not sufficient, as some architectures (e.g., Intel
> > processors) have non-zero cache_leaves() before allocation.
> > 
> > Dereferencing NULL cacheinfo can occur in update_per_cpu_data_slice_size().
> > This function iterates over all online CPUs. However, a CPU may have come
> > online recently, but its cacheinfo may not have been allocated yet.
> > 
> > While here, remove an unnecessary indentation in allocate_cache_info().
> > 
> > Reviewed-by: Andreas Herrmann <aherrmann@xxxxxxx>
> > Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
> > Reviewed-by: Radu Rendec <rrendec@xxxxxxxxxx>
> > Reviewed-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> > Tested-by: Andreas Herrmann <aherrmann@xxxxxxx>
> > Fixes: 6539cffa9495 ("cacheinfo: Add arch specific early level initializer")
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> 
> Reviewed-by: Andreas Herrmann <aherrmann@xxxxxxx>

Thank you!