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

[Ricardo Neri]

On Wed, Sep 11, 2024 at 04:08:44PM +0200, Borislav Petkov wrote:
> 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.

Thank you for your review Borislav!

Yes, both patches address issues during CPU hotplug (both NULL-pointer
dereference). However, IHMO, they are separate issues. Patch 1/4 fixes
a missing allocation check. Patch 2/4 causes the allocation to happen in
case early allocation is not used.

If I did not convince you, I am happy to merge together patches 1 and 2.

Thanks and BR,
Ricardo