Re: [PATCH] fs/resctrl: Fix MBM events being unconditionally enabled in mbm_event mode

["Moger, Babu" <bmoger@amd.com>]

Hi Reinette,

On 10/14/2025 6:09 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 10/14/25 3:45 PM, Moger, Babu wrote:
>> On 10/14/2025 3:57 PM, Reinette Chatre wrote:
>>> On 10/14/25 10:43 AM, Babu Moger wrote:
> 
> 
>>>>> Yes. I saw the issues. It fails to mount in my case with panic trace.
>>>
>>> (Just to ensure that there is not anything else going on) Could you please confirm if the panic is from
>>> mon_add_all_files()->mon_event_read()->mon_event_count()->__mon_event_count()->resctrl_arch_reset_rmid()
>>> that creates the MBM event files during mount and then does the initial read of RMID to determine the
>>> starting count?
>>
>> It happens just before that (at mbm_cntr_get). We have not allocated d->cntr_cfg for the counters.
>> ===================Panic trace =================================
>>
>> 349.330416] BUG: kernel NULL pointer dereference, address: 0000000000000008
>> [  349.338187] #PF: supervisor read access in kernel mode
>> [  349.343914] #PF: error_code(0x0000) - not-present page
>> [  349.349644] PGD 10419f067 P4D 0
>> [  349.353241] Oops: Oops: 0000 [#1] SMP NOPTI
>> [  349.357905] CPU: 45 UID: 0 PID: 3449 Comm: mount Not tainted 6.18.0-rc1+ #120 PREEMPT(voluntary)
>> [  349.367803] Hardware name: AMD Corporation PURICO/PURICO, BIOS RPUT1003E 12/11/2024
>> [  349.376334] RIP: 0010:mbm_cntr_get+0x56/0x90
>> [  349.381096] Code: 45 8d 41 fe 83 f8 01 77 3d 8b 7b 50 85 ff 7e 36 49 8b 84 24 f0 04 00 00 45 31 c0 eb 0d 41 83 c0 01 48 83 c0 10 44 39 c7 74 1c <48> 3b 50 08 75 ed 3b 08 75 e9 48 83 c4 10 44 89 c0 5b 41 5c 41 5d
>> [  349.402037] RSP: 0018:ff56bba58655f958 EFLAGS: 00010246
>> [  349.407861] RAX: 0000000000000000 RBX: ffffffff9525b900 RCX: 0000000000000002
>> [  349.415818] RDX: ffffffff95d526a0 RSI: ff1f5d52517c1800 RDI: 0000000000000020
>> [  349.423774] RBP: ff56bba58655f980 R08: 0000000000000000 R09: 0000000000000001
>> [  349.431730] R10: ff1f5d52c616a6f0 R11: fffc6a2f046c3980 R12: ff1f5d52517c1800
>> [  349.439687] R13: 0000000000000001 R14: ffffffff95d526a0 R15: ffffffff9525b968
>> [  349.447635] FS:  00007f17926b7800(0000) GS:ff1f5d59d45ff000(0000) knlGS:0000000000000000
>> [  349.456659] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [  349.463064] CR2: 0000000000000008 CR3: 0000000147afe002 CR4: 0000000000771ef0
>> [  349.471022] PKRU: 55555554
>> [  349.474033] Call Trace:
>> [  349.476755]  <TASK>
>> [  349.479091]  ? kernfs_add_one+0x114/0x170
>> [  349.483560]  rdtgroup_assign_cntr_event+0x9b/0xd0
>> [  349.488795]  rdtgroup_assign_cntrs+0xab/0xb0
>> [  349.493553]  rdt_get_tree+0x4be/0x770
>> [  349.497623]  vfs_get_tree+0x2e/0xf0
>> [  349.501508]  fc_mount+0x18/0x90
>> [  349.505007]  path_mount+0x360/0xc50
>> [  349.508884]  ? putname+0x68/0x80
>> [  349.512479]  __x64_sys_mount+0x124/0x150
>> [  349.516848]  x64_sys_call+0x2133/0x2190
>> [  349.521123]  do_syscall_64+0x74/0x970
>>
>> ==================================================================
> 
> Thank you for capturing this. This is a different trace but it confirms that it is the
> same root cause. Specifically, event is enabled after the state it depends on is (not) allocated
> during domain online.
> 

Yes. Thanks

Here is the changelog.

x86,fs/resctrl: Fix BUG with mbm_event mode when MBM events are disabled

The following BUG is encountered when mounting the resctrl filesystem 
after booting a system with X86_FEATURE_ABMC support and the kernel 
parameter 'rdt=!mbmtotal,!mbmlocal'.
  
===========================================================================
[  349.330416] BUG: kernel NULL pointer dereference, address: 
0000000000000008
[  349.338187] #PF: supervisor read access in kernel mode
[  349.343914] #PF: error_code(0x0000) - not-present page
[  349.349644] PGD 10419f067 P4D 0
[  349.353241] Oops: Oops: 0000 [#1] SMP NOPTI
[  349.357905] CPU: 45 UID: 0 PID: 3449 Comm: mount Not tainted
                    6.18.0-rc1+ #120 PREEMPT(voluntary)
[  349.367803] Hardware name: AMD Corporation
[  349.376334] RIP: 0010:mbm_cntr_get+0x56/0x90
[  349.381096] Code: 45 8d 41 fe 83 f8 01 77 3d 8b 7b 50 85 ff 7e 36 49 
8b 84 24 f0 04 00 00 45 31 c0 eb 0d 41 83 c0 01 48 83 c0 10 44 39 c7 74 
1c <48> 3b 50 08 75 ed 3b 08 75 e9 48 83 c4 10 44 89 c0 5b 41 5c 41 5d
[  349.402037] RSP: 0018:ff56bba58655f958 EFLAGS: 00010246
[  349.407861] RAX: 0000000000000000 RBX: ffffffff9525b900 RCX: 
0000000000000002
[  349.415818] RDX: ffffffff95d526a0 RSI: ff1f5d52517c1800 RDI: 
0000000000000020
[  349.423774] RBP: ff56bba58655f980 R08: 0000000000000000 R09: 
0000000000000001
[  349.431730] R10: ff1f5d52c616a6f0 R11: fffc6a2f046c3980 R12: 
ff1f5d52517c1800
[  349.439687] R13: 0000000000000001 R14: ffffffff95d526a0 R15: 
ffffffff9525b968
[  349.447635] FS:  00007f17926b7800(0000) GS:ff1f5d59d45ff000(0000)
                     knlGS:0000000000000000
[  349.456659] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  349.463064] CR2: 0000000000000008 CR3: 0000000147afe002 CR4: 
0000000000771ef0
[  349.471022] PKRU: 55555554
[  349.474033] Call Trace:
[  349.476755]  <TASK>
[  349.479091]  ? kernfs_add_one+0x114/0x170
[  349.483560]  rdtgroup_assign_cntr_event+0x9b/0xd0
[  349.488795]  rdtgroup_assign_cntrs+0xab/0xb0
[  349.493553]  rdt_get_tree+0x4be/0x770
[  349.497623]  vfs_get_tree+0x2e/0xf0
[  349.501508]  fc_mount+0x18/0x90
[  349.505007]  path_mount+0x360/0xc50
[  349.508884]  ? putname+0x68/0x80
[  349.512479]  __x64_sys_mount+0x124/0x150

When mbm_event mode is enabled, it implicitly enables both MBM total and
local events. However, specifying the kernel parameter
"rdt=!mbmtotal,!mbmlocal" disables these events during resctrl 
initialization. As a result, related data structures, such as 
rdt_mon_domain::mbm_states, cntr_cfg, and 
rdt_hw_mon_domain::arch_mbm_states are not allocated. This
leads to a BUG when the user attempts to mount the resctrl filesystem,
which tries to access these un-allocated structures.


Fix the issue by adding a dependency on X86_FEATURE_CQM_MBM_TOTAL and
X86_FEATURE_CQM_MBM_LOCAL for X86_FEATURE_ABMC to be enabled. This is
acceptable for now, as X86_FEATURE_ABMC currently implies support for 
MBM total and local events. However, this dependency should be revisited 
and removed in the future to decouple feature handling more cleanly.

Fixes: 13390861b426e ("x86,fs/resctrl: Detect Assignable Bandwidth 
Monitoring feature details")
Co-developed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Babu Moger <babu.moger@amd.com>

====================================================

thanks
Babu