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[109.81.80.247]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45eb6d5e484sm24638766f8f.30.2026.05.25.02.07.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 25 May 2026 02:07:52 -0700 (PDT) Date: Mon, 25 May 2026 11:07:50 +0200 From: Michal Hocko To: Shakeel Butt Cc: Andrew Morton , Johannes Weiner , Roman Gushchin , Muchun Song , Harry Yoo , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] memcg: use round-robin victim selection in refill_stock Message-ID: References: <20260521223751.3794625-1-shakeel.butt@linux.dev> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260521223751.3794625-1-shakeel.butt@linux.dev> On Thu 21-05-26 15:37:51, Shakeel Butt wrote: > Harry Yoo reported that get_random_u32_below() is not safe to call in > the nmi context and memcg charge draining can happen in nmi context. > > More specifically get_random_u32_below() is neither reentrant- nor > NMI-safe: it acquires a per-cpu local_lock via local_lock_irqsave() on > the batched_entropy_u32 state. An NMI that lands on a CPU mid-update of > the ChaCha batch state and recurses into the random subsystem would > corrupt that state. The memcg_stock local_trylock prevents re-entry > on the percpu stock itself, but cannot protect an unrelated > subsystem's per-cpu lock. > > Replace the random pick with a per-cpu round-robin counter stored in > memcg_stock_pcp and serialized by the same local_trylock that already > guards cached[] and nr_pages[]. No atomics, no random calls, no extra > locks needed. > > Fixes: f735eebe55f8f ("memcg: multi-memcg percpu charge cache") > Reported-by: Harry Yoo > Closes: https://lore.kernel.org/4e20f643-6983-4b6e-b12d-c6c4eb20ae0c@kernel.org/ > Signed-off-by: Shakeel Butt Acked-by: Michal Hocko Thanks! > --- > mm/memcontrol.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 0eb50e639f0a..6392a2704441 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2031,6 +2031,7 @@ struct memcg_stock_pcp { > > struct work_struct work; > unsigned long flags; > + uint8_t drain_idx; > }; > > static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = { > @@ -2214,7 +2215,9 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages) > if (!success) { > i = empty_slot; > if (i == -1) { > - i = get_random_u32_below(NR_MEMCG_STOCK); > + i = stock->drain_idx++; > + if (stock->drain_idx == NR_MEMCG_STOCK) > + stock->drain_idx = 0; > drain_stock(stock, i); > } > css_get(&memcg->css); > -- > 2.53.0-Meta -- Michal Hocko SUSE Labs