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From: Shakeel Butt To: Johannes Weiner Cc: Andrew Morton , Michal Hocko , Roman Gushchin , Muchun Song , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team , stable@vger.kernel.org Subject: Re: [PATCH] memcg: drain obj stock on cpu hotplug teardown Message-ID: References: <20250310230934.2913113-1-shakeel.butt@linux.dev> <20250311153032.GB1211411@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250311153032.GB1211411@cmpxchg.org> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: A3F43140013 X-Rspamd-Server: rspam03 X-Stat-Signature: iokkx4jz9fz6kt897qhfyadxet54nt1r X-HE-Tag: 1741708685-29637 X-HE-Meta: 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 R55xEG0+ mMk/0ZzHaeuuewF2joXzsyzKljFjxB7RDe0qUDlza4IyzLEjRrOoyJDruakyfdB9t7/Xk7PauazYfY8I+gEOETRFSXRfjQCSj5xFcBciGA6EtEkqddhdSZmzkv9Kzug1iLqAaUdQyIk8GqKK+hV64tV+PGu4tK8jsPRmsevbe0AswEbv1wkWCYcXGuefp5zOQg8x9OpgbWX4rNdo= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > > Cc: > > 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. Thanks for taking a look.