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

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

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