Re: [PATCH v3 1/9] fs/resctrl: Fix MPAM Partid parsing errors by preserving CDP state during umount

[Zeng Heng <zengheng4@huawei.com>]

On 2026/3/21 12:11, Zeng Heng wrote:
> Hi Ben,
> 
> On 2026/3/21 1:07, Ben Horgan wrote:
>> Hi Zeng,
>>
>> On 3/17/26 13:21, Zeng Heng wrote:
>>> This patch fixes a pre-existing issue in the resctrl filesystem teardown
>>> sequence where premature clearing of cdp_enabled could lead to MPAM 
>>> Partid
>>> parsing errors.
>>>
>>> The closid to partid conversion logic inherently depends on the global
>>> cdp_enabled state. However, rdt_disable_ctx() clears this flag early in
>>> the umount path, while free_rmid() operations will reference after that.
>>> This creates a window where partid parsing operates with inconsistent 
>>> CDP
>>> state, potentially make monitor reads with wrong partid mapping.
>>>
>>> Additionally, rmid_entry remaining in limbo between mount sessions may
>>> trigger potential partid out-of-range errors, leading to MPAM fault
>>> interrupts and subsequent MPAM disablement.
>>>
>>> Reorder rdt_kill_sb() to delay rdt_disable_ctx() until after
>>> rmdir_all_sub() and resctrl_fs_teardown() complete. This ensures
>>> all rmid-related operations finish with correct CDP state.
>>>
>>> Introduce rdt_flush_limbo() to flush and cancel limbo work before the
>>> filesystem teardown completes. An alternative approach would be to 
>>> cancel
>>
>> The code looks correct but it does introduce a subtle change of 
>> behaviour which
>> may or may not be acceptable. A busy rmid may now be allocated after 
>> remount.
>> Clean rmids were never guaranteed, e.g. when a domain goes offline, 
>> but this
>> weakens the guarantee.
> 
> Yes, this would indeed weaken MPAM's guarantee for clean RMIDs.
> 
> Hopefully, no one is doing this in production, repeatedly switching
> resctrl mount modes while monitoring workloads (which sounds more like
> testing to me), and still expecting strict guarantees of clean RMID
> allocation.
> 
>>
>>> limbo work on umount and restart it on remount with remaked bitmap.
>>> However, this would require substantial changes in the resctrl layer to
>>> handle CDP state transitions across mount sessions, which is beyond the
>>> scope of the reqpartid feature work this patchset focuses on. The 
>>> current
>>
>> Another option to consider is whether limbo could be replaced by 
>> checking whether
>> an rmid is busy at allocation.
>>
>> Do your changes here to resctrl_arch_rmid_idx_encode() have an impact 
>> on how
>> limbo works?
> 
> 
> In follow-up patches, resctrl_arch_rmid_idx_encode() also needs to
> depend on the CDP state because it needs to check out the intpartid and
> reqpartid. Between remount sessions, RMIDs residing in limbo also have a
> parsing error issue.
> 
> 

For this reason, had to make this patch as a prerequisite fix in the
patch series.


Best Regards,
Zeng Heng