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d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Cxt2uEcbDsT/cVcss+Oed++7USVk9BRG70PSqUV2sRA=; b=i5rD1ZXQ20MW9szges7afpeU7FdQa9yW9IiRk9jSQEByI8hrD6i2by0Yf7jOc+pIu/ HCYdZykcal9lpb5joFVYmdY2P11kRk3mBHRtk0/x6IyFDBr+962DJNAm4jT2qdd1xxsG gkI1KEFoAYcg3D2x+r/5NPiGJ1lit/AFDEYoRdOal7gTvVnB59G08r1YDrKtxEcM3vq/ pUAaCUHcduxWMxjRXwKViZCtoE9TAM9KwwabGilBiuhlYMWtg03X6lDd1E92igOS6p6j wG+8GvdyLmcPp3GfiLbbbj3vYTBJlKZFnqD//o8lMSpQ0M/oBvXj5Ez4AggtU0hc164W 0gyA== X-Gm-Message-State: AOAM533LhTut6J+ohPQbGGlOMvZnj2k4BRnmIccRK6Q0wo2NCdurAdfH Jbd9RM3v2MpkksosSUhIMnOj2GEGwm2atFGPfQfNbg== X-Google-Smtp-Source: ABdhPJymSM8rAJqKJqvQgojVsV7KVaqMaDPb49Py0OCI7oQ+4nknrXkY/Mk29hxHQ9wU+twofcB4rQhyzFE0gK9E3ow= X-Received: by 2002:a81:78c9:0:b0:2eb:469b:e788 with SMTP id t192-20020a8178c9000000b002eb469be788mr1158786ywc.34.1650065066420; Fri, 15 Apr 2022 16:24:26 -0700 (PDT) MIME-Version: 1.0 References: <20220407031525.2368067-1-yuzhao@google.com> <20220407031525.2368067-9-yuzhao@google.com> <20220411191621.0378467ad99ebc822d5ad005@linux-foundation.org> <20220414185654.e7150bcbe859e0dd4b9c61af@linux-foundation.org> <20220415121521.764a88dda55ae8c676ad26b0@linux-foundation.org> <20220415143220.cc37b0b0a368ed2bf2a821f8@linux-foundation.org> In-Reply-To: From: Jesse Barnes Date: Fri, 15 Apr 2022 16:24:14 -0700 Message-ID: Subject: Re: [page-reclaim] Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks To: Linus Torvalds Cc: Yu Zhao , Andrew Morton , Justin Forbes , Stephen Rothwell , Linux-MM , Andi Kleen , Aneesh Kumar , Barry Song <21cnbao@gmail.com>, Catalin Marinas , Dave Hansen , Hillf Danton , Jens Axboe , Johannes Weiner , Jonathan Corbet , Matthew Wilcox , Mel Gorman , Michael Larabel , Michal Hocko , Mike Rapoport , Rik van Riel , Vlastimil Babka , Will Deacon , Ying Huang , Linux ARM , "open list:DOCUMENTATION" , linux-kernel , Kernel Page Reclaim v2 , "the arch/x86 maintainers" , Brian Geffon , Jan Alexander Steffens , Oleksandr Natalenko , Steven Barrett , Suleiman Souhlal , Daniel Byrne , Donald Carr , =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= , Konstantin Kharlamov , Shuang Zhai , Sofia Trinh , Vaibhav Jain Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 5EF6A180008 X-Stat-Signature: kkjnt8hf7ou9ax3mcmoue8znnbeisnc7 Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=o9JWs2Fy; spf=pass (imf06.hostedemail.com: domain of jsbarnes@google.com designates 209.85.128.171 as permitted sender) smtp.mailfrom=jsbarnes@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1650065067-30268 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: On Fri, Apr 15, 2022 at 4:04 PM Linus Torvalds wrote: > > On Fri, Apr 15, 2022 at 3:58 PM Yu Zhao wrote: > > > > BUG_ONs are harmful but problems that trigger them would be > > presummingly less penetrating to the user base; on the other hand, > > from my experience working with some testers (ordinary users), they > > ignore WARN_ON_ONCEs until the kernel crashes. > > I don't understand your argument. > > First you say that VM_BUG_ON() is only for VM developers. > > Then you say "some testers (ordinary users) ignore WARN_ON_ONCEs until > the kernel crashes". > > So which is it? > > VM developers, or ordinary users? > > Honestly, if a VM developer is ignoring a WARN_ON_ONCE() from the VM > subsystem, I don't even know what to say. > > And for ordinary users, a WARN_ON_ONCE() is about a million times > better, becasue: > > - the machine will hopefully continue working, so they can report the warning > > - even when they don't notice them, distros tend to have automated > reporting infrastructure > > That's why I absolutely *DETEST* those stupid BUG_ON() cases - they > will often kill the machine with nasty locks held, resulting in a > completely undebuggable thing that never gets reported. > > Yes, you can be careful and only put BUG_ON() in places where recovery > is possible. But even then, they have no actual _advantages_ over just > a WARN_ON_ONCE. Generally agreed, and not to belabor this relatively small issue, but in some environments like cloud or managed client deployments, a crash can actually be preferable so we can get a dump, reboot the machine, and get things going again for the application or user, then debug offline. So having the flexibility to do that in those situations is helpful. And there, a full crash dump is better than just a log report with the WARN info, since debugging may be easier with all the kernel memory. Jesse