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Tue, 5 Jul 2022 12:07:16 +0000 (UTC) X-FDA: 79652920872.22.A80D989 Received: from out30-42.freemail.mail.aliyun.com (out30-42.freemail.mail.aliyun.com [115.124.30.42]) by imf11.hostedemail.com (Postfix) with ESMTP id 9883140002 for ; Tue, 5 Jul 2022 12:07:14 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R201e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045168;MF=guanghuifeng@linux.alibaba.com;NM=1;PH=DS;RN=21;SR=0;TI=SMTPD_---0VISK1LP_1657022828; Received: from 30.225.28.170(mailfrom:guanghuifeng@linux.alibaba.com fp:SMTPD_---0VISK1LP_1657022828) by smtp.aliyun-inc.com; Tue, 05 Jul 2022 20:07:09 +0800 Message-ID: <5d044fdd-a61a-d60f-d294-89e17de37712@linux.alibaba.com> Date: Tue, 5 Jul 2022 20:07:07 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation To: Will Deacon , Ard Biesheuvel Cc: baolin.wang@linux.alibaba.com, catalin.marinas@arm.com, akpm@linux-foundation.org, david@redhat.com, jianyong.wu@arm.com, james.morse@arm.com, quic_qiancai@quicinc.com, christophe.leroy@csgroup.eu, jonathan@marek.ca, mark.rutland@arm.com, thunder.leizhen@huawei.com, anshuman.khandual@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, rppt@kernel.org, geert+renesas@glider.be, linux-mm@kvack.org, yaohongbo@linux.alibaba.com, alikernel-developer@linux.alibaba.com References: <20220704103523.GC31437@willie-the-truck> <73f0c53b-fd17-c5e9-3773-1d71e564eb50@linux.alibaba.com> <20220704111402.GA31553@willie-the-truck> <4accaeda-572f-f72d-5067-2d0999e4d00a@linux.alibaba.com> <20220704131516.GC31684@willie-the-truck> <2ae1cae0-ee26-aa59-7ed9-231d67194dce@linux.alibaba.com> <20220704142313.GE31684@willie-the-truck> <6977c692-78ca-5a67-773e-0389c85f2650@linux.alibaba.com> <20220704163815.GA32177@willie-the-truck> <20220705095231.GB552@willie-the-truck> From: "guanghui.fgh" In-Reply-To: <20220705095231.GB552@willie-the-truck> Content-Type: text/plain; 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spf=pass (imf11.hostedemail.com: domain of guanghuifeng@linux.alibaba.com designates 115.124.30.42 as permitted sender) smtp.mailfrom=guanghuifeng@linux.alibaba.com X-Stat-Signature: gdnbfi3en3iq7h195h6k99pzp44tq88t X-Rspamd-Queue-Id: 9883140002 X-Rspam-User: Authentication-Results: imf11.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=alibaba.com; spf=pass (imf11.hostedemail.com: domain of guanghuifeng@linux.alibaba.com designates 115.124.30.42 as permitted sender) smtp.mailfrom=guanghuifeng@linux.alibaba.com X-Rspamd-Server: rspam06 X-HE-Tag: 1657022834-476087 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: 在 2022/7/5 17:52, Will Deacon 写道: > On Mon, Jul 04, 2022 at 07:09:23PM +0200, Ard Biesheuvel wrote: >> On Mon, 4 Jul 2022 at 18:38, Will Deacon wrote: >>> >>> On Mon, Jul 04, 2022 at 10:34:07PM +0800, guanghui.fgh wrote: >>>> Thanks. >>>> >>>> 在 2022/7/4 22:23, Will Deacon 写道: >>>>> On Mon, Jul 04, 2022 at 10:11:27PM +0800, guanghui.fgh wrote: >> ... >>>>>> Namely, it's need to use non block/section mapping for crashkernel mem >>>>>> before shringking. >>>>> >>>>> Well, yes, but we can change arch_kexec_[un]protect_crashkres() not to do >>>>> that if we're leaving the thing mapped, no? >>>>> >>>> I think we should use arch_kexec_[un]protect_crashkres for crashkernel mem. >>>> >>>> Because when invalid crashkernel mem pagetable, there is no chance to rd/wr >>>> the crashkernel mem by mistake. >>>> >>>> If we don't use arch_kexec_[un]protect_crashkres to invalid crashkernel mem >>>> pagetable, there maybe some write operations to these mem by mistake which >>>> may cause crashkernel boot error and vmcore saving error. >>> >>> I don't really buy this line of reasoning. The entire main kernel is >>> writable, so why do we care about protecting the crashkernel so much? The >>> _code_ to launch the crash kernel is writable! If you care about preventing >>> writes to memory which should not be writable, then you should use >>> rodata=full. >>> >> >> This is not entirely true - the core kernel text and rodata are >> remapped r/o in the linear map, whereas all module code and rodata are >> left writable when rodata != full. > > Yes, sorry, you're quite right. The kernel text is only writable if > rodata=off. > > But I still think it makes sense to protect the crashkernel only if > rodata=full (which is the default on arm64) as this allows us to rely > on page mappings and I think fits well with what we do for modules. > >> But the conclusion is the same, imo: if you can't be bothered to >> protect a good chunk of the code and rodata that the kernel relies on, >> why should the crashkernel be treated any differently? > > Thanks. > > Will Thanks. 1.The rodata full is harm to the performance and has been disabled in-house. 2.When using crashkernel with rodata non full, the kernel also will use non block/section mapping which cause high d-TLB miss and degrade performance greatly. This patch fix it to use block/section mapping as far as possible. bool can_set_direct_map(void) { return rodata_full || debug_pagealloc_enabled(); } map_mem: if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)) flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; 3.When rodata full is disabled, crashkernel also need protect(keep arch_kexec_[un]protect_crashkres using). I think crashkernel should't depend on radata full(Maybe other architecture don't support radata full now). Thanks.