Re: [syzbot] [mm?] WARNING in __mod_zone_page_state

[Hugh Dickins <hughd@google.com>]

On Sat, 29 Aug 2026, Shakeel Butt wrote:
> On Sat, Aug 29, 2026 at 08:26:02PM -0700, Hugh Dickins wrote:
...
> > 
> > Thanks for looking into this, Shakeel, but I don't think complicating
> > __munlock_folio() is at all the right fix. This is peculiar to the use
> > by mlock_drain_remote(), isn't it? Which is not taking the usual local_lock
> > because the CPU is going offline. I would say, just take the local_lock in
> > mlock_drain_remote(), but (I haven't read the history) for all I know,
> > there may be PREEMPT_RT reasons why that would be completely wrong.
> > 
> > Hugh
> 
> Thanks Hugh, I will explore the local_lock approach.

Please do. But we may need input from Sebastian. So far as I can see,
page_alloc_cpu_dead()'s neighbouring use of lru_add_drain_cpu(cpu)
would suffer from the exact same issue, there are __counts there too.
Maybe syzbot has not discovered that yet, or maybe I'm confused.

(But you'll understand that I don't particularly welcome a reorg of
the local_locking around the lru_add_drains at the moment; and there's
at least one among them which takes advantage of the fbatch local_lock
to lock something else too.)

Oh for the good old days when we were allowed to say preempt_disable()!

> BTW I simplified the
> fix to the following. is this still making things more complicated?

That is less distracting than your first one, but it's still not the
right fix: the right fix is to have the function called under the
proper conditions in all cases.

> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index efa6716e4dfb..fa30ffed76ab 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -141,11 +141,16 @@ static struct lruvec *__munlock_folio(struct folio *folio, struct lruvec *lruvec
>  
>  munlock:
>  	if (folio_test_clear_mlocked(folio)) {
> -		__zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages);
> +		/*
> +		 * This runs both with and without the lruvec lock held, and
> +		 * mlock_drain_remote() reaches it fully preemptible, so use
> +		 * the accessors that serialize themselves.

I'm very far from being a good advisor on PREEMPT_RT,
but I think that comment about lruvec lock would be wrong there.

Hugh

> +		 */
> +		zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages);
>  		if (isolated || !folio_test_unevictable(folio))
> -			__count_vm_events(UNEVICTABLE_PGMUNLOCKED, nr_pages);
> +			count_vm_events(UNEVICTABLE_PGMUNLOCKED, nr_pages);
>  		else
> -			__count_vm_events(UNEVICTABLE_PGSTRANDED, nr_pages);
> +			count_vm_events(UNEVICTABLE_PGSTRANDED, nr_pages);
>  	}
>  
>  	/* folio_evictable() has to be checked *after* clearing Mlocked */
> --