Re: [PATCH] sched/psi: initialize *flags in psi_memstall_enter when PSI is disabled

[Mashiro Chen]

Hi Johannes,

Good question. You're right that KMSAN's stack tracking persisting
across page reuse boundaries is arguably a tool limitation. That said,
I think fixing it on the PSI side is still reasonable:

psi_memstall_enter() takes a pointer parameter with an implicit contract:
if the caller passes &flags, they expect *flags to be initialized upon
return. The current early-return silently violates that contract by
leaving *flags uninitialized, even though the value is never actually used
functionally.

The fix is essentially free (we're already in the early-return path) and
makes the contract explicit. You're right that the original patch lacked
a comment explaining this, I should have added:

    /* Initialize to 0 even in psi_disabled case to honor the
     * implicit API contract that *flags is initialized on return.
     * psi_memstall_leave() also returns early when psi_disabled
     * and does not read *flags, so this is zero-cost. */
    *flags = 0;
    return;

That said, if you prefer this stays in KMSAN (e.g., treating stack
variables as out-of-scope once their frame returns), I'm happy to drop
the patch and redirect the effort there instead.

Thanks for the thoughtful review feedback.

Best,
Mashiro Chen

On 4/8/26 23:10, Johannes Weiner wrote:
> On Sun, Apr 05, 2026 at 01:50:44PM +0800, Mashiro Chen wrote:
> > When PSI is disabled, psi_memstall_enter() returns early without
> > writing to *flags, leaving the caller's local variable uninitialized.
> > psi_memstall_leave() also returns early when PSI is disabled and does
> > not read *flags, so the uninitialized value is never used functionally.
> >
> > However, KMSAN tracks the shadow and origin metadata per physical
> > address. When a kernel stack page is subsequently reused, a new object
> > at the same address inherits the stale KMSAN shadow from the old
> > uninitialized pflags, causing spurious uninit-value reports in
> > unrelated code paths such as __flush_smp_call_function_queue().
> >
> > Initialize *flags to 0 in the psi_disabled early-return path to
> > prevent the stale shadow from escaping the callers' stack frames.
> How is this not a kmsan bug?
>
> I don't think putting an unexpected init into unrelated code with no
> comment is an appropriate fix for what seems like a false positive in
> this tool.