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[195.250.132.146]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a729d7c9ad6sm54586966b.209.2024.06.27.05.28.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 27 Jun 2024 05:28:14 -0700 (PDT) Date: Thu, 27 Jun 2024 14:28:13 +0200 From: Michal Hocko To: Yosry Ahmed Cc: Xiu Jianfeng , hannes@cmpxchg.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, akpm@linux-foundation.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next] mm: memcg: remove redundant seq_buf_has_overflowed() Message-ID: References: <20240626094232.2432891-1-xiujianfeng@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 3D37D18001E X-Stat-Signature: wh8f1eggz6kpdshyd67p1onaekf9w1cs X-HE-Tag: 1719491295-446879 X-HE-Meta: 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 uc1PZV9c VR1Q31FcJLMcN4bkZGWIa6A3Kr5HvbLctakI3vQUXq91DFgIUKiTL6mOfgLFuncS593l95inr1nmQWAxF54OdZVXqix4og/OaB5klwSFIc7JqOLXTwnXoxuHzNm404oQydsh7Enys0LBlr0k7s95ksesmo6h4H2zRbC3zwQItu1Je/cQ05R5R+c4J0IBX5YBVGnEoip4CkU2PBHToT9+JJTL+BgCYTe1Ec7dequ08srEYoqV4uAsGsvhdJjbMRcpqhoqyZ4mT0s27vbDSFagpXM/bRACUShhQfHcGqqLYDJE8/JPZ0DygXIKv8gGT0+i5upGNiPNArE8oC5PIYwEdW54hY/ySaCIeSIcSkzvmkH9gaWiqpfFE59v9iYQivvWQ2JFuEz8f2II2HSSix2LUqmoxdOUnDJfBF4zK X-Bogosity: Ham, tests=bogofilter, spamicity=0.014901, 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 Thu 27-06-24 05:00:18, Yosry Ahmed wrote: > On Thu, Jun 27, 2024 at 4:56 AM Michal Hocko wrote: > > > > On Thu 27-06-24 04:33:50, Yosry Ahmed wrote: > > > On Thu, Jun 27, 2024 at 12:13 AM Michal Hocko wrote: > > > > > > > > On Wed 26-06-24 09:42:32, Xiu Jianfeng wrote: > > > > > Both the end of memory_stat_format() and memcg_stat_format() will call > > > > > WARN_ON_ONCE(seq_buf_has_overflowed()). However, memory_stat_format() > > > > > is the only caller of memcg_stat_format(), when memcg is on the default > > > > > hierarchy, seq_buf_has_overflowed() will be executed twice, so remove > > > > > the reduntant one. > > > > > > > > Shouldn't we rather remove both? Are they giving us anything useful > > > > actually? Would a simpl pr_warn be sufficient? Afterall all we care > > > > about is to learn that we need to grow the buffer size because our stats > > > > do not fit anymore. It is not really important whether that is an OOM or > > > > cgroupfs interface path. > > > > > > Is it possible for userspace readers to break if the stats are > > > incomplete? > > > > They will certainly get an imprecise picture. Sufficient to break I > > dunno. > > If some stats go completely missing and a parser expects them to > always be there, I think they may break. If they break, we will eventually learn about that with or without warning. It is true that WARN* is so vocal that people/tooling might just report that even without breakage but that to me sounds like abusing WARNING. There were times when this was not a big deal but now when WARN* are getting CVEs because panic_on_warn this useful debugging tool has become a new BUG on. -- Michal Hocko SUSE Labs