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

[Mashiro Chen <mashiro.chen@mailbox.org>]

Hi Johannes,

You're right on both counts. The 'opaque channel' framing makes it
clear there's no meaningful API contract being violated here -- the
caller is not supposed to interpret *flags at all.

And yes, your second point is exactly the real issue: once a stack
frame returns, its local variables should be considered dead. KMSAN
tracking that shadow across page reuse into an unrelated frame is the
actual bug.

I'll drop this patch. The correct fix is in KMSAN -- it should treat
stack slots as out-of-scope once their owning frame returns, rather
than letting stale shadow metadata escape into subsequent users of
the same physical address.

Thanks for the clear explanation.

Best,
Mashiro Chen

On 4/9/26 00:40, Johannes Weiner wrote:
> On Thu, Apr 09, 2026 at 12:14:50AM +0800, Mashiro Chen wrote:
>> 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 caller has no expectations towards the contents of *flags and no
> business reading or manipulating them. It's an opaque channel that
> lets _enter() communicate with _leave().
>
>> 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.
> It sounds to me like this would be a good thing to fix regardless of
> what psi is doing here. Even if psi initialized it to some value that
> is meaningful to psi - that value is totally random, and for all
> intents and purposes "uninitialized", from the view of a subsequent
> user of that stack slot?