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Tue, 5 Jul 2022 02:44:55 +0000 (UTC) X-FDA: 79651503750.14.0011D61 Received: from out30-57.freemail.mail.aliyun.com (out30-57.freemail.mail.aliyun.com [115.124.30.57]) by imf16.hostedemail.com (Postfix) with ESMTP id 2E0FE180020 for ; Tue, 5 Jul 2022 02:44:52 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R311e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045170;MF=guanghuifeng@linux.alibaba.com;NM=1;PH=DS;RN=21;SR=0;TI=SMTPD_---0VIPhkH4_1656989083; Received: from 30.225.28.170(mailfrom:guanghuifeng@linux.alibaba.com fp:SMTPD_---0VIPhkH4_1656989083) by smtp.aliyun-inc.com; Tue, 05 Jul 2022 10:44:45 +0800 Message-ID: Date: Tue, 5 Jul 2022 10:44:43 +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 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, ardb@kernel.org, linux-mm@kvack.org, yaohongbo@linux.alibaba.com, alikernel-developer@linux.alibaba.com References: <1656777473-73887-1-git-send-email-guanghuifeng@linux.alibaba.com> <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> From: "guanghui.fgh" In-Reply-To: <20220704163815.GA32177@willie-the-truck> Content-Type: text/plain; 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cv=none; b=mRUgjTUaf3I2C/qNF1rgKXvHPpmpy3SJ/b7DDpDdaoANYpqk+tqRUVWMe0x+Sr5BZpIAW7 cjncqYldlZl3pU5+KLUx+oEheDnae/uDP59JE/Jk98ShjrAyRftGG7WSSTRYW9u1AmHH+u LFVlyVFgTlFIXpysy5gYGiDecnT6Njg= X-Rspam-User: X-Rspamd-Queue-Id: 2E0FE180020 Authentication-Results: imf16.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=alibaba.com; spf=pass (imf16.hostedemail.com: domain of guanghuifeng@linux.alibaba.com designates 115.124.30.57 as permitted sender) smtp.mailfrom=guanghuifeng@linux.alibaba.com X-Stat-Signature: zqnhpbj1ys1azsothtjz9ypqxwapaw1f X-Rspamd-Server: rspam08 X-HE-Tag: 1656989092-708771 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 0:38, Will Deacon 写道: > 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: >>>> 在 2022/7/4 21:15, Will Deacon 写道: >>>>> On Mon, Jul 04, 2022 at 08:05:59PM +0800, guanghui.fgh wrote: >>>>>>>> 1.Quoted messages from arch/arm64/mm/init.c >>>>>>>> >>>>>>>> "Memory reservation for crash kernel either done early or deferred >>>>>>>> depending on DMA memory zones configs (ZONE_DMA) -- >>>>>>>> >>>>>>>> In absence of ZONE_DMA configs arm64_dma_phys_limit initialized >>>>>>>> here instead of max_zone_phys(). This lets early reservation of >>>>>>>> crash kernel memory which has a dependency on arm64_dma_phys_limit. >>>>>>>> Reserving memory early for crash kernel allows linear creation of block >>>>>>>> mappings (greater than page-granularity) for all the memory bank rangs. >>>>>>>> In this scheme a comparatively quicker boot is observed. >>>>>>>> >>>>>>>> If ZONE_DMA configs are defined, crash kernel memory reservation >>>>>>>> is delayed until DMA zone memory range size initialization performed in >>>>>>>> zone_sizes_init(). The defer is necessary to steer clear of DMA zone >>>>>>>> memory range to avoid overlap allocation. >>>>>>>> >>>>>>>> [[[ >>>>>>>> So crash kernel memory boundaries are not known when mapping all bank memory >>>>>>>> ranges, which otherwise means not possible to exclude crash kernel range >>>>>>>> from creating block mappings so page-granularity mappings are created for >>>>>>>> the entire memory range. >>>>>>>> ]]]" >>>>>>>> >>>>>>>> Namely, the init order: memblock init--->linear mem mapping(4k mapping for >>>>>>>> crashkernel, requirinig page-granularity changing))--->zone dma >>>>>>>> limit--->reserve crashkernel. >>>>>>>> So when enable ZONE DMA and using crashkernel, the mem mapping using 4k >>>>>>>> mapping. >>>>>>> >>>>>>> Yes, I understand that is how things work today but I'm saying that we may >>>>>>> as well leave the crashkernel mapped (at block granularity) if >>>>>>> !can_set_direct_map() and then I think your patch becomes a lot simpler. >>>>>> >>>>>> But Page-granularity mapppings are necessary for crash kernel memory range >>>>>> for shrinking its size via /sys/kernel/kexec_crash_size interfac(Quoted from >>>>>> arch/arm64/mm/init.c). >>>>>> So this patch split block/section mapping to 4k page-granularity mapping for >>>>>> crashkernel mem. >>>>> >>>>> Why? I don't see why the mapping granularity is relevant at all if we >>>>> always leave the whole thing mapped. >>>>> >>>> There is another reason. >>>> >>>> When loading crashkernel finish, the do_kexec_load will use >>>> arch_kexec_protect_crashkres to invalid all the pagetable for crashkernel >>>> mem(protect crashkernel mem from access). >>>> >>>> arch_kexec_protect_crashkres--->set_memory_valid--->...--->apply_to_pmd_range >>>> >>>> In the apply_to_pmd_range, there is a judement： BUG_ON(pud_huge(*pud)). And >>>> if the crashkernel use block/section mapping, there will be some error. >>>> >>>> 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. > > Will Thanks. 1.I think the normal kernel mem can be writeable or readonly 2.But the normal kernel should't access(read/write) crashkernel mem after the crashkernel is loaded to the mem.(despite the normal kernel write/read access protect) So invalid pagetable for crashkernel mem is needed. With this method, it can guarantee the usability of the crashkernel when occuring emergency and generating vmcore.