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Thu, 17 Apr 2025 07:48:39 -0700 (PDT) Date: Thu, 17 Apr 2025 10:48:35 -0400 From: Johannes Weiner To: Muchun Song Cc: mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, akpm@linux-foundation.org, david@fromorbit.com, zhengqi.arch@bytedance.com, yosry.ahmed@linux.dev, nphamcs@gmail.com, chengming.zhou@linux.dev, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, hamzamahfooz@linux.microsoft.com, apais@linux.microsoft.com Subject: Re: [PATCH RFC 02/28] mm: memcontrol: use folio_memcg_charged() to avoid potential rcu lock holding Message-ID: <20250417144835.GE780688@cmpxchg.org> References: <20250415024532.26632-1-songmuchun@bytedance.com> <20250415024532.26632-3-songmuchun@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@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: <20250415024532.26632-3-songmuchun@bytedance.com> On Tue, Apr 15, 2025 at 10:45:06AM +0800, Muchun Song wrote: > If a folio isn't charged to the memory cgroup, holding an rcu read lock > is needless. Users only want to know its charge status, so use > folio_memcg_charged() here. > > Signed-off-by: Muchun Song > --- > mm/memcontrol.c | 11 ++++------- > 1 file changed, 4 insertions(+), 7 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 61488e45cab2..0fc76d50bc23 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -797,20 +797,17 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx, > void __lruvec_stat_mod_folio(struct folio *folio, enum node_stat_item idx, > int val) > { > - struct mem_cgroup *memcg; > pg_data_t *pgdat = folio_pgdat(folio); > struct lruvec *lruvec; > > - rcu_read_lock(); > - memcg = folio_memcg(folio); > - /* Untracked pages have no memcg, no lruvec. Update only the node */ > - if (!memcg) { > - rcu_read_unlock(); > + if (!folio_memcg_charged(folio)) { > + /* Untracked pages have no memcg, no lruvec. Update only the node */ > __mod_node_page_state(pgdat, idx, val); > return; > } > > - lruvec = mem_cgroup_lruvec(memcg, pgdat); > + rcu_read_lock(); > + lruvec = mem_cgroup_lruvec(folio_memcg(folio), pgdat); > __mod_lruvec_state(lruvec, idx, val); > rcu_read_unlock(); Hm, but untracked pages are the rare exception. It would seem better for that case to take the rcu_read_lock() unnecessarily, than it is to look up folio->memcg_data twice in the fast path?