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

[Hao Li]

On Mon, Nov 02, 2026 at 02:35:40PM +0000, Harry Yoo wrote:
> Here I added bit more context and adjusted comment. End result:
> 
> mm/slab: don't queue the page cache work under krcp->lock
> 
> Commit 56292e8609e3 ("rcu/tree: Defer kvfree_rcu() allocation to a
> clean context") deferred page allocations to a workqueue. To avoid
> deadlocks with the scheduler, it used hrtimer to queue the work instead
> of queueing it directly.
> 
> Commit d0bfa8b3c411 ("kvfree_rcu: Release a page cache under memory
> pressure") later added a backoff mechanism under memory pressure, but
> did not consider that queue_delayed_work() with delay = 0 immediately
> queues the work and introduced the deadlock commit 56292e8609e3 tried to
> avoid.
> 
> 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")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> Link: https://patch.msgid.link/20261028134647.3176080-2-zhanxusheng1024@xxxxxxxxx
> [ harry@xxxxxxxxxx: add more context to changelog and adjust comment ]
> Signed-off-by: Harry Yoo (Meta) <harry@xxxxxxxxxx>
> ---
>  mm/slab_common.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)

Reviewed-by: Hao Li <hao.li@xxxxxxxxx>

-- 
Thanks,
Hao