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AJvYcCVdLkhtbQ+IS4eaqBa/fJC0wFryh76SXQE9RG2HKrh4VRK8RYsct1BDfWCqcSFVrsY5ZR/vE3D8V4sBeMAe955dNcU= X-Gm-Message-State: AOJu0YzIhj0qsi4gyjzVu2ZRoQQVh7Ztf4IteqaDKAE4aVULBSg5rDnt dWZch/H2Xu+f5nw6OlC0xsF912jQJ/ml79Cy+lnbe6eCo+6/21zLV/iL/BFogj9oQitA0DQ5tcw nAVhle0GORQtA6DYvgNTIeUjfcW8= X-Google-Smtp-Source: AGHT+IH79rROqNcavSMXsf+wfYIKr3IoiUhbmwvAnK/aIP2yy4o+3XclT5fVjjkOfSFC9OkXp5msM0Y696izruyL5u8= X-Received: by 2002:a25:ae47:0:b0:dfa:ac81:38e8 with SMTP id 3f1490d57ef6-dfe65e7ee45mr3107880276.6.1718244820063; Wed, 12 Jun 2024 19:13:40 -0700 (PDT) MIME-Version: 1.0 References: <20240608155316.451600-1-flintglass@gmail.com> <20240608155316.451600-2-flintglass@gmail.com> In-Reply-To: From: Takero Funaki Date: Thu, 13 Jun 2024 11:13:29 +0900 Message-ID: Subject: Re: [PATCH v1 1/3] mm: zswap: fix global shrinker memcg iteration To: Yosry Ahmed Cc: Nhat Pham , Johannes Weiner , Chengming Zhou , Jonathan Corbet , Andrew Morton , Domenico Cerasuolo , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Shakeel Butt Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 06041C0002 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 8fcj8utsn8n4pcexdjwt39uh8kshnux6 X-HE-Tag: 1718244820-581826 X-HE-Meta: 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 ZkF8L9Wt nC8v8CdVJRusBg7/eJ9S4aThN7KDdC+HOaWP29wBJ6T4d7XS2Dbp+DN+izDko9gakfx8+4hd5mRirYvOZZcVOshxVToCfllFrRoJ1XP8bv+f1w82BsKvMkabQANGkjFWPqesjOAepfscddcPT92a2H/8MujxAmA+AX1L6F2qFQ/YIYjn6LPFwRYYMCCZv5TUI2OcQpcXNeGepwJJffZ5pkXCNuGWBgfD4TLeybYyt8wKFT7hoUXBsI3H1B9VBgZctJ0/SDjTldTD6UHmafu619JjyURBZvWux3CVpNSzH2O+avlfxpxxIaoRZ92GlJM89eSIEu0yD4KYurP73jqwdGcmHhE5a33DXVFP5miv5aWZ9hyURdQgXIz/CN6FbGoMhcF68ZegzEhnhQ720fKwcj5i9iw== 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: 2024=E5=B9=B46=E6=9C=8813=E6=97=A5(=E6=9C=A8) 3:28 Yosry Ahmed : > > On Wed, Jun 12, 2024 at 11:16=E2=80=AFAM Takero Funaki wrote: > > > > 2024=E5=B9=B46=E6=9C=8812=E6=97=A5(=E6=B0=B4) 3:26 Nhat Pham : > > > > > > > > As I have noted in v0, I think this is unnecessary and makes it more = confusing. > > > > > > > Does spin_lock() ensure that compiler optimizations do not remove > > memory access to an external variable? I think we need to use > > READ_ONCE/WRITE_ONCE for shared variable access even under a spinlock. > > For example, > > https://elixir.bootlin.com/linux/latest/source/mm/mmu_notifier.c#L234 > > In this example, it seems like mmu_interval_set_seq() updates > interval_sub->invalidate_seq locklessly using WRITE_ONCE(). I think > this is why READ_ONCE() is required in that particular case. > > > > > isn't this a common use case of READ_ONCE? > > ```c > > bool shared_flag =3D false; > > spinlock_t flag_lock; > > > > void somefunc(void) { > > for (;;) { > > spin_lock(&flag_lock); > > /* check external updates */ > > if (READ_ONCE(shared_flag)) > > break; > > /* do something */ > > spin_unlock(&flag_lock); > > } > > spin_unlock(&flag_lock); > > } > > ``` > > Without READ_ONCE, the check can be extracted from the loop by optimiza= tion. > > According to Documentation/memory-barriers.txt, lock acquiring > functions are implicit memory barriers. Otherwise, the compiler would > be able to pull any memory access outside of the lock critical section > and locking wouldn't be reliable. Ah, I understand now. The implicit barrier is sufficient as long as all memory access occurs within the lock. It=E2=80=99s a fundamental rule o= f locking=E2=80=94facepalm. I misread a module code, like in the link, where a lockless write could invade a critical section. My example was in the opposite direction, just wrong. Thank you so much for clarifying my misunderstanding. For now checking the patch, I suppose the locking mechanism itself is not affected by my misunderstanding of READ_ONCE. The corrected version of the cleaner should be: ```c void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg) { /* lock out zswap shrinker walking memcg tree */ spin_lock(&zswap_shrink_lock); if (zswap_next_shrink =3D=3D memcg) { do { zswap_next_shrink =3D mem_cgroup_iter(NULL, zswap_next_shrink, NULL); spin_unlock(&zswap_shrink_lock); spin_lock(&zswap_shrink_lock); if (!zswap_next_shrink) break; } while (!mem_cgroup_online(zswap_next_shrink)); } spin_unlock(&zswap_shrink_lock); } ``` Should we have a separate patch to fix the leak scenario?