Re: [PATCH v2 6/7] mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()

[Uladzislau Rezki]

On Fri, Feb 21, 2025 at 06:28:49PM +0100, Vlastimil Babka wrote:
> On 2/21/25 17:30, Keith Busch wrote:
> > On Wed, Aug 07, 2024 at 12:31:19PM +0200, Vlastimil Babka wrote:
> >> We would like to replace call_rcu() users with kfree_rcu() where the
> >> existing callback is just a kmem_cache_free(). However this causes
> >> issues when the cache can be destroyed (such as due to module unload).
> >> 
> >> Currently such modules should be issuing rcu_barrier() before
> >> kmem_cache_destroy() to have their call_rcu() callbacks processed first.
> >> This barrier is however not sufficient for kfree_rcu() in flight due
> >> to the batching introduced by a35d16905efc ("rcu: Add basic support for
> >> kfree_rcu() batching").
> >> 
> >> This is not a problem for kmalloc caches which are never destroyed, but
> >> since removing SLOB, kfree_rcu() is allowed also for any other cache,
> >> that might be destroyed.
> >> 
> >> In order not to complicate the API, put the responsibility for handling
> >> outstanding kfree_rcu() in kmem_cache_destroy() itself. Use the newly
> >> introduced kvfree_rcu_barrier() to wait before destroying the cache.
> >> This is similar to how we issue rcu_barrier() for SLAB_TYPESAFE_BY_RCU
> >> caches, but has to be done earlier, as the latter only needs to wait for
> >> the empty slab pages to finish freeing, and not objects from the slab.
> >> 
> >> Users of call_rcu() with arbitrary callbacks should still issue
> >> rcu_barrier() before destroying the cache and unloading the module, as
> >> kvfree_rcu_barrier() is not a superset of rcu_barrier() and the
> >> callbacks may be invoking module code or performing other actions that
> >> are necessary for a successful unload.
> >> 
> >> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> >> ---
> >>  mm/slab_common.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >> 
> >> diff --git a/mm/slab_common.c b/mm/slab_common.c
> >> index c40227d5fa07..1a2873293f5d 100644
> >> --- a/mm/slab_common.c
> >> +++ b/mm/slab_common.c
> >> @@ -508,6 +508,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
> >>  	if (unlikely(!s) || !kasan_check_byte(s))
> >>  		return;
> >>  
> >> +	/* in-flight kfree_rcu()'s may include objects from our cache */
> >> +	kvfree_rcu_barrier();
> >> +
> >>  	cpus_read_lock();
> >>  	mutex_lock(&slab_mutex);
> > 
> > This patch appears to be triggering a new warning in certain conditions
> > when tearing down an nvme namespace's block device. Stack trace is at
> > the end.
> > 
> > The warning indicates that this shouldn't be called from a
> > WQ_MEM_RECLAIM workqueue. This workqueue is responsible for bringing up
> > and tearing down block devices, so this is a memory reclaim use AIUI.
> > I'm a bit confused why we can't tear down a disk from within a memory
> > reclaim workqueue. Is the recommended solution to simply remove the WQ
> > flag when creating the workqueue?
> 
> I think it's reasonable to expect a memory reclaim related action would
> destroy a kmem cache. Mateusz's suggestion would work around the issue, but
> then we could get another surprising warning elsewhere. Also making the
> kmem_cache destroys async can be tricky when a recreation happens
> immediately under the same name (implications with sysfs/debugfs etc). We
> managed to make the destroying synchronous as part of this series and it
> would be great to keep it that way.
> 
> >   ------------[ cut here ]------------
> >   workqueue: WQ_MEM_RECLAIM nvme-wq:nvme_scan_work is flushing !WQ_MEM_RECLAIM events_unbound:kfree_rcu_work
> 
> Maybe instead kfree_rcu_work should be using a WQ_MEM_RECLAIM workqueue? It
> is after all freeing memory. Ulad, what do you think?
> 
We reclaim memory, therefore WQ_MEM_RECLAIM seems what we need.
AFAIR, there is an extra rescue worker, which can really help
under a low memory condition in a way that we do a progress.

Do we have a reproducer of mentioned splat?

--
Uladzislau Rezki