From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0748BCCA473 for ; Wed, 6 Jul 2022 10:04:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232442AbiGFKEi (ORCPT ); Wed, 6 Jul 2022 06:04:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43270 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231824AbiGFKEf (ORCPT ); Wed, 6 Jul 2022 06:04:35 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D210224F00 for ; Wed, 6 Jul 2022 03:04:34 -0700 (PDT) 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 61A68B81BA7 for ; Wed, 6 Jul 2022 10:04:33 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7825BC3411C; Wed, 6 Jul 2022 10:04:28 +0000 (UTC) Date: Wed, 6 Jul 2022 11:04:24 +0100 From: Catalin Marinas To: Mike Rapoport 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: <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; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 05, 2022 at 11:45:40PM +0300, Mike Rapoport wrote: > 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. We can reserve the region just before unmapping to avoid new allocations for the page tables but we can't do much about pages already allocated prior to calling remap_crashkernel(). -- Catalin From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CB279C433EF for ; Wed, 6 Jul 2022 10:14:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=w5tlIkZk7BxIumidsAPgcti8HOn3+6ziC9Ty0pymMy0=; b=hqRsclMnN7GLtc gouRQ7n3ATZ0D4WSRk2ZgUnojFR9XW9Bp6BBAHzn5dfZ1zZNVYYSq7MWikm3x9JE1VOhd2d405Olu FqfzCJuT+84c8yOxv+wiaI8K8zMfFxAvStKVXyvpJ7cfeDZ9NaBwwPDJ+VStn8Oxm9x78gFq2Cayo isCIC0hIJ8kNMHvENPCJb0jWsxk/TVfmumcX4m6WcI1kS2JnwQNE3kwjbE06cUgXnju/BH185+uGm +uj8rRgIMr6gG4NUq8Mc76iXRn3v+Wpp1y4eml3A/xMviFd883F85DMrhvoo5691SDwORh4k656E1 EneH+flKeLq17SsSdUSw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1o921y-008FgC-J8; Wed, 06 Jul 2022 10:13:16 +0000 Received: from dfw.source.kernel.org ([139.178.84.217]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1o91tc-0087b8-0j for linux-arm-kernel@lists.infradead.org; Wed, 06 Jul 2022 10:04:39 +0000 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 dfw.source.kernel.org (Postfix) with ESMTPS id B65E761D82; Wed, 6 Jul 2022 10:04:32 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7825BC3411C; Wed, 6 Jul 2022 10:04:28 +0000 (UTC) Date: Wed, 6 Jul 2022 11:04:24 +0100 From: Catalin Marinas To: Mike Rapoport 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: <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-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220706_030436_187909_F2DCC83A X-CRM114-Status: GOOD ( 28.54 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jul 05, 2022 at 11:45:40PM +0300, Mike Rapoport wrote: > 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. We can reserve the region just before unmapping to avoid new allocations for the page tables but we can't do much about pages already allocated prior to calling remap_crashkernel(). -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel