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Mon, 29 Aug 2022 14:31:58 +0000 (UTC) X-FDA: 79852869516.03.FDA37B0 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf07.hostedemail.com (Postfix) with ESMTP id D2E334002B for ; Mon, 29 Aug 2022 14:31:57 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 74719B80FA3; Mon, 29 Aug 2022 14:31:56 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 24B28C433C1; Mon, 29 Aug 2022 14:31:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1661783515; bh=KXyavkK9jmchKS7BM+QIziwxtNYM2+lbxNBUUd3PHyM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=pA/7OOQUY7XbeE1en6O+TKcX7NVXmCgSUo0LFicvnYUdsGNwOxmQyJYCf3dPiZGPq x4Scu4HcCVz/Gsqim3s73gcMswgBBN+ZePPcmKiUd/rjj8Zlz36TpZOjeLNO+yhcVa z5VNUAVpZw6cQkITOzrXbR6az7CHVc1x/IGCgl63fLTbUJ0R9Q1KXjjMoOM9QvesJg 43Tewm0guxDD6hw4uI+87G4+Y/MoaIgKizEfB7U2xx952E6nntJyEC6UArWKCTyk/e 0RnVCmmOg+D1u0LzKx8dh5pnr4FHFtqLsX3pT0U8Qm9Q5FnMuSRxQjmcN9fDMo7yv7 vM9/oezVpYc3Q== Date: Mon, 29 Aug 2022 17:31:37 +0300 From: Mike Rapoport To: Baoquan He Cc: linux-arm-kernel@lists.infradead.org, Ard Biesheuvel , kexec@lists.infradead.org, Catalin Marinas , Guanghui Feng , Mark Rutland , Mike Rapoport , Will Deacon , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mike Rapoport Subject: Re: [PATCH 0/5] arm64/mm: remap crash kernel with base pages even if rodata_full disabled Message-ID: References: <20220819041156.873873-1-rppt@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; 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cv=none; b=5kBa0YAZF9FAUnQ3WTEiljNvPLcKnMIDjEPJPvExwZKXEdEkY9hf4bvOHl1tsNDUgqPPpF gmVL3odvHmbntUYj2lUeHiXmhi6rCoC78IiYF5fUhf2tfjcjWrW3iOX51DQNdHKA7SXyd/ ci3aTeQP9i81fculYL9y1WayhD7MyqU= X-Rspam-User: X-Rspamd-Queue-Id: D2E334002B X-Stat-Signature: w5dyn9pp5sqrdsfqpn6i4xiahghi9sk8 Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b="pA/7OOQU"; spf=pass (imf07.hostedemail.com: domain of rppt@kernel.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=rppt@kernel.org; dmarc=pass (policy=none) header.from=kernel.org X-Rspamd-Server: rspam08 X-HE-Tag: 1661783517-699973 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 Sun, Aug 28, 2022 at 04:37:29PM +0800, Baoquan He wrote: > On 08/25/22 at 10:48am, Mike Rapoport wrote: > ...... > > > > There were several rounds of discussion how to remap with base pages only > > > > the crash kernel area, the latest one here: > > > > > > > > https://lore.kernel.org/all/1656777473-73887-1-git-send-email-guanghuifeng@linux.alibaba.com > > > > > > > > and this is my attempt to allow having both large pages in the linear map > > > > and protection for the crash kernel memory. > > > > > > > > For server systems it is important to protect crash kernel memory for > > > > post-mortem analysis, and for that protection to work the crash kernel > > > > memory should be mapped with base pages in the linear map. > > > > > > > > On the systems with ZONE_DMA/DMA32 enabled, crash kernel reservation > > > > happens after the linear map is created and the current code forces using > > > > base pages for the entire linear map, which results in performance > > > > degradation. > > > > > > > > These patches enable remapping of the crash kernel area with base pages > > > > while keeping large pages in the rest of the linear map. > > > > > > > > The idea is to align crash kernel reservation to PUD boundaries, remap that > > > > PUD and then free the extra memory. > > > > > > Hi Mike, > > > > > > Thanks for the effort to work on this issue. While I have to say this > > > isnt's good because it can only be made relying on a prerequisite that > > > there's big enough memory. If on a system, say 2G memory, it's not easy > > > to succeed on getting one 1G memory. While we only require far smaller > > > region than 1G, e.g about 200M which should be easy to get. So the way > > > taken in this patchset is too quirky and will cause regression on > > > systemswith small memory. This kind of sytems with small memory exists > > > widely on virt guest instance. > > > > I don't agree there is a regression. If the PUD-aligned allocation fails, > > there is a fallback to the allocation of the exact size requested for crash > > kernel. This allocation just won't get protected. > > Sorry, I misunderstood it. I just went through the log and didn't > look into codes. > > But honestly, if we accept the fallback which doesn't do the protection, > we should be able to take off the protection completely, right? > Otherwise, the reservation code is a little complicated. We don't do protection of the crash kernel for most architectures supporting kexec ;-) My goal was to allow large systems with ZONE_DMA/DMA32 have block mappings in the linear map and crash kernel protection without breaking backward compatibility for the existing systems. > > Also please note, that the changes are only for the case when user didn't > > force base-size pages in the linear map, so anything that works now will > > work the same way with this set applied. > > > > > The crashkernel reservation happens after linear map because the > > > reservation needs to know the dma zone boundary, arm64_dma_phys_limit. > > > If we can deduce that before bootmem_init(), the reservation can be > > > done before linear map. I will make an attempt on that. If still can't > > > be accepted, we would like to take off the crashkernel region protection > > > on arm64 for now. > > > > I doubt it would be easy because arm64_dma_phys_limit is determined after > > parsing of the device tree and there might be memory allocations of > > possibly unmapped memory during the parsing. > > I have sent out the patches with an attempt, it's pretty straightforward > and simple. Because arm64 only has one exception, namely Raspberry Pi 4, > on which some peripherals can only address 30bit range. That is a corner > case, to be honest. And kdump is a necessary feature on server, but may > not be so expected on Raspberry Pi 4, a system for computer education > and hobbyists. And kdump only cares whether the dump target devices can > address 32bit range, namely storage device or network card on server. > If finally confirmed that storage devices can only address 30bit range > on Raspberry Pi 4, people still can have crashkernel=xM@yM method to > reserve crashkernel regions. I hope you are right and Raspberry Pi 4 is the only system that limits DMA'able range to 30 bits. But with diversity of arm64 chips and boards I won't be surprised that there are other variants with a similar problem. > Thanks > Baoquan > -- Sincerely yours, Mike.