Re: [PATCH v6 33/42] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point

[Reinette Chatre <reinette.chatre@intel.com>]

Hi James,

On 3/6/25 11:28 AM, James Morse wrote:
> Hi Reinette,
> 
> On 01/03/2025 02:35, Reinette Chatre wrote:
>> On 2/28/25 11:54 AM, James Morse wrote:
>>> On 20/02/2025 04:42, Reinette Chatre wrote:
>>>> On 2/7/25 10:18 AM, James Morse wrote:
>>>>> resctrl_exit() was intended for use when the 'resctrl' module was unloaded.
>>>>> resctrl can't be built as a module, and the kernfs helpers are not exported
>>>>> so this is unlikely to change. MPAM has an error interrupt which indicates
>>>>> the MPAM driver has gone haywire. Should this occur tasks could run with
>>>>> the wrong control values, leading to bad performance for important tasks.
>>>>> The MPAM driver needs a way to tell resctrl that no further configuration
>>>>> should be attempted.
>>>>>
>>>>> Using resctrl_exit() for this leaves the system in a funny state as
>>>>> resctrl is still mounted, but cannot be un-mounted because the sysfs
>>>>> directory that is typically used has been removed. Dave Martin suggests
>>>>> this may cause systemd trouble in the future as not all filesystems
>>>>> can be unmounted.
>>>>>
>>>>> Add calls to remove all the files and directories in resctrl, and
>>>>> remove the sysfs_remove_mount_point() call that leaves the system
>>>>> in a funny state. When triggered, this causes all the resctrl files
>>>>> to disappear. resctrl can be unmounted, but not mounted again.
>>>
>>>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>>> index 6e30283358d4..424622d2f959 100644
>>>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>>> @@ -4371,9 +4375,12 @@ int __init resctrl_init(void)
>>>>>  
>>>>
>>>> Could you please add the kerneldoc you proposed in
>>>> https://lore.kernel.org/lkml/f2ecb501-bc65-49a9-903d-80ba1737845f@arm.com/ ?
>>>
>>> Huh. The way that is indented means I copied it out the file - I'm not sure went wrong
>>> there. Thanks for fishing out the link!
>>>
>>>
>>>>>  void __exit resctrl_exit(void)
>>>>>  {
>>>>> +	mutex_lock(&rdtgroup_mutex);
>>>>> +	rdtgroup_destroy_root();
>>>>> +	mutex_unlock(&rdtgroup_mutex);
>>>>> +
>>>>>  	debugfs_remove_recursive(debugfs_resctrl);
>>>>>  	unregister_filesystem(&rdt_fs_type);
>>>>> -	sysfs_remove_mount_point(fs_kobj, "resctrl");
>>>>>  
>>>>>  	resctrl_mon_resource_exit();
>>>>>  }
>>>>
>>>> It is difficult for me to follow the kernfs reference counting required
>>>> to make this work. Specifically, the root kn is "destroyed" here but it
>>>> is required to stick around until unmount when the rest of the files
>>>> are removed.
>>>
>>> This drops resctrl's reference to all of the files, which would make the files disappear.
>>> unmount is what calls kernfs_kill_sb(), which gets rid of the root of the filesystem.
>>
>> My concern is mostly with the kernfs_remove() calls in the rdt_kill_sb()->rmdir_all_sub()
>> flow. For example:
>> 	kernfs_remove(kn_info);
>> 	kernfs_remove(kn_mongrp);
>> 	kernfs_remove(kn_mondata);
>>
>> As I understand the above require the destroyed root to still be around.
> 
> Right - because rdt_get_tree() has these global pointers into the hierarchy, but doesn't
> take a reference. rmdir_all_sub() relies on always being called before
> rdtgroup_destroy_root().
> 
> The point hack would be for rdtgroup_destroy_root() to NULL out those global pointers, (I
> note they are left dangling) - that would make a subsequent call to rmdir_all_sub() harmless.
> 
> A better fix would be to pull out all the filesystem relevant parts from rdt_kill_sb(),
> make that safe for multiple calls and get resctrl_exit() to call that.
> A call to rdt_kill_sb() after resctrl_exit() would just cleanup the super-block.
> This will leave things in a more predictable state.
> 
> 

Since V7 has been posted already I try to keep things coherent by copying this
exchange and responded to you there [1].

Reinette

[1] https://lore.kernel.org/lkml/053d8a62-022b-4bf8-8e47-651e7c3a2d59@intel.com/