Re: [PATCH] memcg: drain obj stock on cpu hotplug teardown

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 11, 2025 at 08:57:54AM -0700, Shakeel Butt wrote:
> On Tue, Mar 11, 2025 at 11:30:32AM -0400, Johannes Weiner wrote:
> > On Mon, Mar 10, 2025 at 04:09:34PM -0700, Shakeel Butt wrote:
> > > Currently on cpu hotplug teardown, only memcg stock is drained but we
> > > need to drain the obj stock as well otherwise we will miss the stats
> > > accumulated on the target cpu as well as the nr_bytes cached. The stats
> > > include MEMCG_KMEM, NR_SLAB_RECLAIMABLE_B & NR_SLAB_UNRECLAIMABLE_B. In
> > > addition we are leaking reference to struct obj_cgroup object.
> > > 
> > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> > > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > > Cc: <stable@vger.kernel.org>
> > 
> > Wow, that's old. Good catch.
> > 
> > > ---
> > >  mm/memcontrol.c | 9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > > 
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 4de6acb9b8ec..59dcaf6a3519 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -1921,9 +1921,18 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
> > >  static int memcg_hotplug_cpu_dead(unsigned int cpu)
> > >  {
> > >  	struct memcg_stock_pcp *stock;
> > > +	struct obj_cgroup *old;
> > > +	unsigned long flags;
> > >  
> > >  	stock = &per_cpu(memcg_stock, cpu);
> > > +
> > > +	/* drain_obj_stock requires stock_lock */
> > > +	local_lock_irqsave(&memcg_stock.stock_lock, flags);
> > > +	old = drain_obj_stock(stock);
> > > +	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
> > > +
> > >  	drain_stock(stock);
> > > +	obj_cgroup_put(old);
> > 
> > It might be better to call drain_local_stock() directly instead. That
> > would prevent a bug of this type to reoccur in the future.
> 
> The issue is drain_local_stock() works on the local cpu stock while here
> we are working on a remote cpu cpu which is dead (memcg_hotplug_cpu_dead
> is in PREPARE section of hotplug teardown which runs after the cpu is
> dead).
> 
> We can safely call drain_stock() on remote cpu stock here but
> drain_obj_stock() is a bit tricky as it can __refill_stock() to local cpu
> stock and can call __mod_objcg_mlstate to flush stats. Both of these
> requires irq disable for NON-RT kernels and thus I added the local_lock
> here.
> 
> Anyways I wanted a simple fix for the backports and in parallel I am
> working on cleaning up all the stock functions as I plan to add multi
> memcg support.

True, it can be refactored separately.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>