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Tue, 5 Jul 2022 20:46:07 +0000 (UTC) X-FDA: 79654228374.29.0AC065D Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) by imf23.hostedemail.com (Postfix) with ESMTP id 19C6F140025 for ; Tue, 5 Jul 2022 20:46:05 +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 sin.source.kernel.org (Postfix) with ESMTPS id 071F7CE1D38; Tue, 5 Jul 2022 20:46:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 131BDC341C7; Tue, 5 Jul 2022 20:45:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1657053961; bh=vV5sEaKKx1MzkRfFumfYYlM3/ThLTBn8xX32cScp1ro=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=PrrYt2wokC2svFhtJCfa8Has+IZKh3ZUcO2lXuaN+RetPSUDog6AtzHWnGNdgJxyR 7c9EjwQfrti5rGmyrlmJ4cFIVN5Nixqf5UIYh6q5w2JlauC4ir+8m1yeVrbw5BhF+0 XLPCp52MVK6dkKxdFQp+0pyXSkqkcFR5FSSoq0hHb9+8/ApZ1+mgqrifoqdNz0jOzh h5AP8QGxlMbjL4GOjRU3IVGVcye4gSEAdkGhkti092PnurdP8XwzBWWtuMUcLDOL1D 1K9awSBbQEWgKecLJmRI1lTMYL9LXnIDZcsQcrrog6ZHvKqV7sMUI8kohF4A9x0MSo w8A/Xk78JeccA== Date: Tue, 5 Jul 2022 23:45:40 +0300 From: Mike Rapoport To: Catalin Marinas Cc: Will Deacon , "guanghui.fgh" , Ard Biesheuvel , baolin.wang@linux.alibaba.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, geert+renesas@glider.be, linux-mm@kvack.org, yaohongbo@linux.alibaba.com, alikernel-developer@linux.alibaba.com Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation Message-ID: References: <6977c692-78ca-5a67-773e-0389c85f2650@linux.alibaba.com> <20220704163815.GA32177@willie-the-truck> <20220705095231.GB552@willie-the-truck> <5d044fdd-a61a-d60f-d294-89e17de37712@linux.alibaba.com> <20220705121115.GB1012@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; 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cv=none; b=TOXyESwDGnzPjF7M4P23+mZ10MLOhcgwHLl94SlpAUplPgPHVmE3bHhr10zDoB6kRGD9r5 /RP6Mgk2CPxVNk1CpUjc+hmvHEvkXEQ2l1b9I/ZvVkI2nYO14fvH6/NJURHkP3a+po1Qme Y2/4xOKWuf0q4ZYdQgYUMKWmG1stjyQ= X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: me7n76tcguoqg61sdq75ozzaj6yun3ua X-Rspamd-Queue-Id: 19C6F140025 Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=PrrYt2wo; dmarc=pass (policy=none) header.from=kernel.org; spf=pass (imf23.hostedemail.com: domain of rppt@kernel.org designates 145.40.73.55 as permitted sender) smtp.mailfrom=rppt@kernel.org X-HE-Tag: 1657053965-955368 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, Jul 05, 2022 at 06:05:01PM +0100, Catalin Marinas wrote: > On Tue, Jul 05, 2022 at 06:57:53PM +0300, Mike Rapoport wrote: > > On Tue, Jul 05, 2022 at 04:34:09PM +0100, Catalin Marinas wrote: > > > On Tue, Jul 05, 2022 at 06:02:02PM +0300, Mike Rapoport wrote: > > > > +void __init remap_crashkernel(void) > > > > +{ > > > > +#ifdef CONFIG_KEXEC_CORE > > > > + phys_addr_t start, end, size; > > > > + phys_addr_t aligned_start, aligned_end; > > > > + > > > > + if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)) > > > > + return; > > > > + > > > > + if (!crashk_res.end) > > > > + return; > > > > + > > > > + start = crashk_res.start & PAGE_MASK; > > > > + end = PAGE_ALIGN(crashk_res.end); > > > > + > > > > + aligned_start = ALIGN_DOWN(crashk_res.start, PUD_SIZE); > > > > + aligned_end = ALIGN(end, PUD_SIZE); > > > > + > > > > + /* Clear PUDs containing crash kernel memory */ > > > > + unmap_hotplug_range(__phys_to_virt(aligned_start), > > > > + __phys_to_virt(aligned_end), false, NULL); > > > > > > What I don't understand is what happens if there's valid kernel data > > > between aligned_start and crashk_res.start (or the other end of the > > > range). > > > > Data shouldn't go anywhere :) > > > > There is > > > > + /* map area from PUD start to start of crash kernel with large pages */ > > + size = start - aligned_start; > > + __create_pgd_mapping(swapper_pg_dir, aligned_start, > > + __phys_to_virt(aligned_start), > > + size, PAGE_KERNEL, early_pgtable_alloc, 0); > > > > and > > > > + /* map area from end of crash kernel to PUD end with large pages */ > > + size = aligned_end - end; > > + __create_pgd_mapping(swapper_pg_dir, end, __phys_to_virt(end), > > + size, PAGE_KERNEL, early_pgtable_alloc, 0); > > > > after the unmap, so after we tear down a part of a linear map we > > immediately recreate it, just with a different page size. > > > > This all happens before SMP, so there is no concurrency at that point. > > That brief period of unmap worries me. The kernel text, data and stack > are all in the vmalloc space but any other (memblock) allocation to this > point may be in the unmapped range before and after the crashkernel > reservation. The interrupts are off, so I think the only allocation and > potential access that may go in this range is the page table itself. But > it looks fragile to me. I agree there are chances there will be an allocation from the unmapped range. We can make sure this won't happen, though. We can cap the memblock allocations with memblock_set_current_limit(aligned_end) or memblock_reserve(algined_start, aligned_end) until the mappings are restored. > -- > Catalin -- Sincerely yours, Mike.