From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755568AbdEEGw7 (ORCPT ); Fri, 5 May 2017 02:52:59 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:35046 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752141AbdEEGw5 (ORCPT ); Fri, 5 May 2017 02:52:57 -0400 Date: Fri, 5 May 2017 08:52:53 +0200 From: Ingo Molnar To: 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 Subject: Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity mapping if available Message-ID: <20170505065253.s5yv7c2cxgfcf4i3@gmail.com> References: <1493862171-8799-1-git-send-email-xlpang@redhat.com> <1493862171-8799-2-git-send-email-xlpang@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1493862171-8799-2-git-send-email-xlpang@redhat.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * 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. 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, 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. )