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

[Zeng Heng]

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