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

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Message-ID: <Z9Bkn63LE4JRB3x7@google.com>
Date: Tue, 11 Mar 2025 16:28:15 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>, stable@...r.kernel.org
Subject: Re: [PATCH] memcg: drain obj stock on cpu hotplug teardown

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@...ux.dev>
> > > Cc: <stable@...r.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.

Really curious to see patches/more details here, I have some ideas here
as well (nothing materialized yet though).

Thanks!