Re: [PATCH 2/2] mm/slab: don't queue the page cache work under krcp->lock

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Oct 28, 2026 at 09:46:47PM +0800, Zhan Xusheng wrote:
> From: Zhan Xusheng <zhanxusheng@xiaomi.com>
> 
> run_page_cache_worker() runs under krcp->lock, from kvfree_call_rcu()
> callers that may hold more, such as set_cpus_allowed_force() under
> pi_lock, so it kicks the refill work from an hrtimer.  Under memory
> pressure it instead queues the work with a delay of
> rcutree.rcu_delay_page_cache_fill_msec, which may be 0, and a zero delay
> queues it at once, taking pool->lock under krcp->lock.
> 
> With the delay set to 0, calling kvfree_rcu() on vmalloc memory under
> pi_lock after a drop_caches makes lockdep report:
> 
>   Chain exists of:
>     &pool->lock --> &p->pi_lock --> krc.lock
> 
> Use the hrtimer when the delay is 0.
> 
> Fixes: d0bfa8b3c411 ("kvfree_rcu: Release a page cache under memory pressure")
> Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
> ---
>  mm/slab_common.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 3729aa7a6156..ef5ff0a20249 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1987,7 +1987,13 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
>  
>  	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
>  			!atomic_xchg(&krcp->work_in_progress, 1)) {
> -		if (atomic_read(&krcp->backoff_page_cache_fill)) {
> +		/*
> +		 * A zero delay would queue the work right here, under
> +		 * krcp->lock and possibly the caller's pi_lock, so let the
> +		 * hrtimer do it.
> +		 */
> +		if (atomic_read(&krcp->backoff_page_cache_fill) &&
> +		    rcu_delay_page_cache_fill_msec) {
>  			queue_delayed_work(rcu_reclaim_wq,
>  				&krcp->page_cache_work,
>  					msecs_to_jiffies(rcu_delay_page_cache_fill_msec));
> -- 
> 2.43.0
> 
> 
Can we just clamp this to 1 - 100 seconds interval?

<snip>
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 7223a7596dab..2c9236236ac9 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -2249,12 +2249,12 @@ void __init kvfree_rcu_init(void)
 			WQ_UNBOUND | WQ_MEM_RECLAIM, 0);
 	WARN_ON(!rcu_reclaim_wq);
 
-	/* Clamp it to [0:100] seconds interval. */
-	if (rcu_delay_page_cache_fill_msec < 0 ||
+	/* Clamp it to [1:100] seconds interval. */
+	if (rcu_delay_page_cache_fill_msec <= 0 ||
 		rcu_delay_page_cache_fill_msec > 100 * MSEC_PER_SEC) {
 
 		rcu_delay_page_cache_fill_msec =
-			clamp(rcu_delay_page_cache_fill_msec, 0,
+			clamp(rcu_delay_page_cache_fill_msec, 1,
 				(int) (100 * MSEC_PER_SEC));
 
 		pr_info("Adjusting rcutree.rcu_delay_page_cache_fill_msec to %d ms.\n",
<snip>

so we forbid zero as value to use?

--
Uladzislau Rezki