From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755858AbdEEHb3 (ORCPT ); Fri, 5 May 2017 03:31:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42778 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751764AbdEEHb2 (ORCPT ); Fri, 5 May 2017 03:31:28 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com DC5C0C04B943 Authentication-Results: ext-mx07.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx07.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=xpang@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com DC5C0C04B943 Reply-To: xlpang@redhat.com Subject: Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity mapping if available References: <1493862171-8799-1-git-send-email-xlpang@redhat.com> <1493862171-8799-2-git-send-email-xlpang@redhat.com> <20170505065253.s5yv7c2cxgfcf4i3@gmail.com> To: Ingo Molnar , Xunlei Pang Cc: linux-kernel@vger.kernel.org, kexec@lists.infradead.org, akpm@linux-foundation.org, Eric Biederman , Dave Young , x86@kernel.org, Borislav Petkov , Thomas Gleixner , Yinghai Lu , "H. Peter Anvin" , Andy Lutomirski From: Xunlei Pang Message-ID: <590C2A96.3090106@redhat.com> Date: Fri, 5 May 2017 15:32:38 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20170505065253.s5yv7c2cxgfcf4i3@gmail.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Fri, 05 May 2017 07:31:28 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/05/2017 at 02:52 PM, Ingo Molnar wrote: > * Xunlei Pang wrote: > >> @@ -122,6 +122,10 @@ static int init_pgtable(struct kimage *image, unsigned long start_pgtable) >> >> level4p = (pgd_t *)__va(start_pgtable); >> clear_page(level4p); >> + >> + if (direct_gbpages) >> + info.direct_gbpages = true; > No, this should be keyed off the CPU feature (X86_FEATURE_GBPAGES) automatically, > not set blindly! AFAICS this patch will crash kexec on any CPU that does not > support gbpages. It should be fine, probe_page_size_mask() already takes care of this: if (direct_gbpages && boot_cpu_has(X86_FEATURE_GBPAGES)) { printk(KERN_INFO "Using GB pages for direct mapping\n"); page_size_mask |= 1 << PG_LEVEL_1G; } else { direct_gbpages = 0; } So if X86_FEATURE_GBPAGES is not supported, direct_gbpages will be set to 0. > > I only noticed this problem after having fixed/enhanced all the changelogs - so > please pick up the new changelog up from the log below. Thanks for the rewrite, it looks better. Regards, Xunlei > > Thanks, > > Ingo > > > ============================> > > Author: Xunlei Pang > > x86/mm: Add support for gbpages to kernel_ident_mapping_init() > > Kernel identity mappings on x86-64 kernels are created in two > ways: by the early x86 boot code, or by kernel_ident_mapping_init(). > > Native kernels (which is the dominant usecase) use the former, > but the kexec and the hibernation code uses kernel_ident_mapping_init(). > > There's a subtle difference between these two ways of how identity > mappings are created, the current kernel_ident_mapping_init() code > creates identity mappings always using 2MB page(PMD level) - while > the native kernel boot path also utilizes gbpages where available. > > This difference is suboptimal both for performance and for memory > usage: kernel_ident_mapping_init() needs to allocate pages for the > page tables when creating the new identity mappings. > > This patch adds 1GB page(PUD level) support to kernel_ident_mapping_init() > to address these concerns. > > The primary advantage would be better TLB coverage/performance, > because we'd utilize 1GB TLBs instead of 2MB ones. > > It is also useful for machines with large number of memory to > save paging structure allocations(around 4MB/TB using 2MB page) > when setting identity mappings for all the memory, after using > 1GB page it will consume only 8KB/TB. > > ( Note that this change alone does not activate gbpages in kexec, > we are doing that in a separate patch. ) >