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 X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B8B22C282C4 for ; Mon, 4 Feb 2019 19:51:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 919A82080A for ; Mon, 4 Feb 2019 19:51:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726865AbfBDTvM (ORCPT ); Mon, 4 Feb 2019 14:51:12 -0500 Received: from mga05.intel.com ([192.55.52.43]:26660 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725854AbfBDTvM (ORCPT ); Mon, 4 Feb 2019 14:51:12 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga105.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 04 Feb 2019 11:51:11 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.56,560,1539673200"; d="scan'208";a="297217646" Received: from ahduyck-desk1.jf.intel.com ([10.7.198.76]) by orsmga005.jf.intel.com with ESMTP; 04 Feb 2019 11:51:11 -0800 Message-ID: <10fe638278abc129eff53779cffb476f4fcbbf64.camel@linux.intel.com> Subject: Re: [RFC PATCH 4/4] mm: Add merge page notifier From: Alexander Duyck To: Dave Hansen , Alexander Duyck , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: rkrcmar@redhat.com, x86@kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, pbonzini@redhat.com, tglx@linutronix.de, akpm@linux-foundation.org Date: Mon, 04 Feb 2019 11:51:11 -0800 In-Reply-To: <33d14370-b47d-5ceb-09c4-41f0d6b33af8@intel.com> References: <20190204181118.12095.38300.stgit@localhost.localdomain> <20190204181558.12095.83484.stgit@localhost.localdomain> <33d14370-b47d-5ceb-09c4-41f0d6b33af8@intel.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.5 (3.28.5-2.fc28) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2019-02-04 at 11:40 -0800, Dave Hansen wrote: > > +void __arch_merge_page(struct zone *zone, struct page *page, > > + unsigned int order) > > +{ > > + /* > > + * The merging logic has merged a set of buddies up to the > > + * KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER. Since that is the case, take > > + * advantage of this moment to notify the hypervisor of the free > > + * memory. > > + */ > > + if (order != KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER) > > + return; > > + > > + /* > > + * Drop zone lock while processing the hypercall. This > > + * should be safe as the page has not yet been added > > + * to the buddy list as of yet and all the pages that > > + * were merged have had their buddy/guard flags cleared > > + * and their order reset to 0. > > + */ > > + spin_unlock(&zone->lock); > > + > > + kvm_hypercall2(KVM_HC_UNUSED_PAGE_HINT, page_to_phys(page), > > + PAGE_SIZE << order); > > + > > + /* reacquire lock and resume freeing memory */ > > + spin_lock(&zone->lock); > > +} > > Why do the lock-dropping on merge but not free? What's the difference? The lock has not yet been acquired in the free path. The arch_free_page call is made from free_pages_prepare, whereas the arch_merge_page call is made from within __free_one_page which has the requirement that the zone lock be taken before calling the function. > This makes me really nervous. You at *least* want to document this at > the arch_merge_page() call-site, and perhaps even the __free_one_page() > call-sites because they're near where the zone lock is taken. Okay, that makes sense. I would probably look at adding the documentation to the arch_merge_page call-site. > The place you are calling arch_merge_page() looks OK to me, today. But, > it can't get moved around without careful consideration. That also > needs to be documented to warn off folks who might move code around. Agreed. > The interaction between the free and merge hooks is also really > implementation-specific. If an architecture is getting order-0 > arch_free_page() notifications, it's probably worth at least documenting > that they'll *also* get arch_merge_page() notifications. If an architecture is getting order-0 notifications then the merge notifications would be pointless since all the pages would be already hinted. I can add documentation that explains that in the case where we are only hinting on non-zero order pages then arch_merge_page should provide hints for when a page is merged above that threshold. > The reason x86 doesn't double-hypercall on those is not broached in the > descriptions. That seems to be problematic. I will add more documentation to address that.