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[209.85.208.172]) by smtp.gmail.com with ESMTPSA id 80sm9538418ljf.38.2020.08.10.14.00.00 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 10 Aug 2020 14:00:01 -0700 (PDT) Received: by mail-lj1-f172.google.com with SMTP id h19so11120148ljg.13 for ; Mon, 10 Aug 2020 14:00:00 -0700 (PDT) X-Received: by 2002:a2e:7615:: with SMTP id r21mr1353407ljc.371.1597093200342; Mon, 10 Aug 2020 14:00:00 -0700 (PDT) MIME-Version: 1.0 References: <20200810145701.129228-1-peterx@redhat.com> <20200810191520.GA132381@xz-x1> In-Reply-To: From: Linus Torvalds Date: Mon, 10 Aug 2020 13:59:44 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2] mm/gup: Allow real explicit breaking of COW To: Peter Xu Cc: Linux-MM , Linux Kernel Mailing List , Andrew Morton , Marty Mcfadden , Andrea Arcangeli , Jann Horn , Christoph Hellwig , Oleg Nesterov , Kirill Shutemov , Jan Kara Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 5EDC91AD1B9 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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 Mon, Aug 10, 2020 at 1:51 PM Linus Torvalds wrote: > > See? That's the whole point. It doesn't _matter_ if you're only > reading the data, without the COW you may be reading the _wrong_ data. Example safe users: - if you hold the mmap_sem the whole time, you're fine - if you only look up the page mapping for some tracing reasons, you're fine - if you can explain some other reason why you don't care that some other process might be changing the data from under you, you might be fine. But the point is, you need to _explain_ it. Not just say "I don't want COW". Linus