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[209.85.167.50]) by smtp.gmail.com with ESMTPSA id k20sm255794lfm.93.2021.09.08.11.13.45 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Sep 2021 11:13:45 -0700 (PDT) Received: by mail-lf1-f50.google.com with SMTP id s10so6122629lfr.11 for ; Wed, 08 Sep 2021 11:13:45 -0700 (PDT) X-Received: by 2002:a05:6512:1112:: with SMTP id l18mr3468117lfg.402.1631124825204; Wed, 08 Sep 2021 11:13:45 -0700 (PDT) MIME-Version: 1.0 References: <20210907195226.14b1d22a07c085b22968b933@linux-foundation.org> <20210908025730.S88ylmikU%akpm@linux-foundation.org> In-Reply-To: <20210908025730.S88ylmikU%akpm@linux-foundation.org> From: Linus Torvalds Date: Wed, 8 Sep 2021 11:13:29 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [patch 079/147] fs/proc/kcore.c: add mmap interface To: Andrew Morton Cc: Alexey Dobriyan , chenying.kernel@bytedance.com, Linux-MM , mm-commits@vger.kernel.org, Mike Rapoport , Muchun Song , zhouchengming@bytedance.com, zhoufeng.zf@bytedance.com Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 54C02F000090 X-Stat-Signature: 1gyed6zscxydnxsqzie76siiishfmssx Authentication-Results: imf17.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=Rwva5mDk; spf=pass (imf17.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.208.178 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org; dmarc=none X-HE-Tag: 1631124828-892076 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 Tue, Sep 7, 2021 at 7:57 PM Andrew Morton wrote: > > After looking at the kcore code, it is found that kcore does not implement > mmap, resulting in frequent context switching triggered by read. > Therefore, we want to add mmap interface to optimize performance. Since > vmalloc and module areas will change with allocation and release, > consistency cannot be guaranteed, so mmap interface only maps KCORE_TEXT > and KCORE_RAM. Honestly, I still hate this patch. The last time people wanted to speed up /dev/kcore accesses, it was all for black-hat reasons and speeding up kernel attacks. And this code just makes me nervous even aside from that, because I do not understand what the heck it's doing. > + if (kern_addr_valid(start)) { > + if (m->type == KCORE_RAM) > + pfn = __pa(start) >> PAGE_SHIFT; > + else if (m->type == KCORE_TEXT) > + pfn = __pa_symbol(start) >> PAGE_SHIFT; Why is "__pa(start)" right in one situation, and "__pa_symbol(start)" in another. So this just makes me go "this is all confusing, dangerous, and the use-case is dubious". Mapping kernel memory is dangerous. The use-cases for it are dubious. The patch isn't obvious. All of that screams "I'll skip this". Linus