[PATCH 1/2] mm/slab: restore the rcutree. prefix of the kvfree_rcu parameters

[PATCH 1/2] mm/slab: restore the rcutree. prefix of the kvfree_rcu parameters

[Zhan Xusheng]

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

The move of kvfree_rcu() to mm/slab_common.c dropped the "rcutree."
MODULE_PARAM_PREFIX, so rcu_min_cached_objs and
rcu_delay_page_cache_fill_msec became slab_common.* and the documented
rcutree.* names are silently ignored.  Restore the prefix; the
slab_common.* names, never documented, stop working.

Fixes: bbe658d65802 ("mm/slab: Move kvfree_rcu() into SLAB")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
 mm/slab_common.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 01c64ef661e7..3729aa7a6156 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1373,6 +1373,10 @@ void __init kvfree_rcu_init(void)
  * per-CPU. Object size is equal to one page. This value
  * can be changed at boot time.
  */
+/* These used to be rcutree parameters; keep their documented names. */
+#undef MODULE_PARAM_PREFIX
+#define MODULE_PARAM_PREFIX "rcutree."
+
 static int rcu_min_cached_objs = 5;
 module_param(rcu_min_cached_objs, int, 0444);
 

base-commit: 65ba8f34ec886ec72b2c07ddc8cf3d815c32869d
-- 
2.43.0

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

[Zhan Xusheng]

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

Re: [PATCH 1/2] mm/slab: restore the rcutree. prefix of the kvfree_rcu parameters

[Harry Yoo]

On Wed, Oct 28, 2026 at 09:46:46PM +0800, Zhan Xusheng wrote:
> From: Zhan Xusheng <zhanxusheng@xiaomi.com>
> 
> The move of kvfree_rcu() to mm/slab_common.c dropped the "rcutree."
> MODULE_PARAM_PREFIX, so rcu_min_cached_objs and
> rcu_delay_page_cache_fill_msec became slab_common.* and the documented
> rcutree.* names are silently ignored.  Restore the prefix; the
> slab_common.* names, never documented, stop working.
> 
> Fixes: bbe658d65802 ("mm/slab: Move kvfree_rcu() into SLAB")

We should add:

Cc: stable@vger.kernel.org

This is a functional regression and in MM we don't automatically
backport fixes to -stable unless without this tag.

> Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
> ---
>  mm/slab_common.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 01c64ef661e7..3729aa7a6156 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1373,6 +1373,10 @@ void __init kvfree_rcu_init(void)
>   * per-CPU. Object size is equal to one page. This value
>   * can be changed at boot time.
>   */
> +/* These used to be rcutree parameters; keep their documented names. */
> +#undef MODULE_PARAM_PREFIX
> +#define MODULE_PARAM_PREFIX "rcutree."

This should be moved to before "This rcu parameter is
runtime-read-only ..." part?

And perhaps let's rephrase the comment, something like:

/* Keep the prefix inherited from RCU */

With that adjusted, otherwise looks good to me, so:
Reviewed-by: Harry Yoo (Meta) <harry@kernel.org>

-- 
Cheers,
Harry / Hyeonggon