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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT 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 D1FC2C32789 for ; Tue, 6 Nov 2018 11:20:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A300720869 for ; Tue, 6 Nov 2018 11:20:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A300720869 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730450AbeKFUpT (ORCPT ); Tue, 6 Nov 2018 15:45:19 -0500 Received: from mga01.intel.com ([192.55.52.88]:17895 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726976AbeKFUpT (ORCPT ); Tue, 6 Nov 2018 15:45:19 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 06 Nov 2018 03:20:36 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.54,471,1534834800"; d="scan'208";a="83694421" Received: from aaronlu.sh.intel.com (HELO intel.com) ([10.239.159.44]) by fmsmga007.fm.intel.com with ESMTP; 06 Nov 2018 03:20:33 -0800 Date: Tue, 6 Nov 2018 19:20:33 +0800 From: Aaron Lu To: Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Andrew Morton , =?utf-8?B?UGF3ZcWC?= Staszewski , Jesper Dangaard Brouer , Eric Dumazet , Tariq Toukan , Ilias Apalodimas , Yoel Caspersen , Mel Gorman , Saeed Mahameed , Michal Hocko , Dave Hansen , Alexander Duyck Subject: Re: [PATCH v2 2/2] mm/page_alloc: use a single function to free page Message-ID: <20181106112033.GB24198@intel.com> References: <20181105085820.6341-1-aaron.lu@intel.com> <20181105085820.6341-2-aaron.lu@intel.com> <20181106053037.GD6203@intel.com> <20181106084746.GA24198@intel.com> <30aa9d1f-d619-c143-3de6-6876029538bc@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <30aa9d1f-d619-c143-3de6-6876029538bc@suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 06, 2018 at 10:32:00AM +0100, Vlastimil Babka wrote: > On 11/6/18 9:47 AM, Aaron Lu wrote: > > On Tue, Nov 06, 2018 at 09:16:20AM +0100, Vlastimil Babka wrote: > >> On 11/6/18 6:30 AM, Aaron Lu wrote: > >>> We have multiple places of freeing a page, most of them doing similar > >>> things and a common function can be used to reduce code duplicate. > >>> > >>> It also avoids bug fixed in one function but left in another. > >>> > >>> Signed-off-by: Aaron Lu > >> > >> Acked-by: Vlastimil Babka > > > > Thanks. > > > >> I assume there's no arch that would run page_ref_sub_and_test(1) slower > >> than put_page_testzero(), for the critical __free_pages() case? > > > > Good question. > > > > I followed the non-arch specific calls and found that: > > page_ref_sub_and_test() ends up calling atomic_sub_return(i, v) while > > put_page_testzero() ends up calling atomic_sub_return(1, v). So they > > should be same for archs that do not have their own implementations. > > x86 seems to distinguish between DECL and SUBL, see Ah right. > arch/x86/include/asm/atomic.h although I could not figure out where does > e.g. arch_atomic_dec_and_test become atomic_dec_and_test to override the > generic implementation. I didn't check that either but I think it will :-) > I don't know if the CPU e.g. executes DECL faster, but objectively it > has one parameter less. Maybe it doesn't matter? No immediate idea. > > Back to your question: I don't know either. > > If this is deemed unsafe, we can probably keep the ref modify part in > > their original functions and only take the free part into a common > > function. > > I guess you could also employ if (__builtin_constant_p(nr)) in > free_the_page(), but the result will be ugly I guess, and maybe not > worth it :) Right I can't make it clean. I think I'll just move the free part a common function and leave the ref decreasing part as is to be safe. Regards, Aaron > >>> --- > >>> v2: move comments close to code as suggested by Dave. > >>> > >>> mm/page_alloc.c | 36 ++++++++++++++++-------------------- > >>> 1 file changed, 16 insertions(+), 20 deletions(-) > >>> > >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c > >>> index 91a9a6af41a2..4faf6b7bf225 100644 > >>> --- a/mm/page_alloc.c > >>> +++ b/mm/page_alloc.c > >>> @@ -4425,9 +4425,17 @@ unsigned long get_zeroed_page(gfp_t gfp_mask) > >>> } > >>> EXPORT_SYMBOL(get_zeroed_page); > >>> > >>> -void __free_pages(struct page *page, unsigned int order) > >>> +static inline void free_the_page(struct page *page, unsigned int order, int nr) > >>> { > >>> - if (put_page_testzero(page)) { > >>> + VM_BUG_ON_PAGE(page_ref_count(page) == 0, page); > >>> + > >>> + /* > >>> + * Free a page by reducing its ref count by @nr. > >>> + * If its refcount reaches 0, then according to its order: > >>> + * order0: send to PCP; > >>> + * high order: directly send to Buddy. > >>> + */ > >>> + if (page_ref_sub_and_test(page, nr)) { > >>> if (order == 0) > >>> free_unref_page(page); > >>> else > >>> @@ -4435,6 +4443,10 @@ void __free_pages(struct page *page, unsigned int order) > >>> } > >>> } > >>> > >>> +void __free_pages(struct page *page, unsigned int order) > >>> +{ > >>> + free_the_page(page, order, 1); > >>> +} > >>> EXPORT_SYMBOL(__free_pages); > >>> > >>> void free_pages(unsigned long addr, unsigned int order) > >>> @@ -4481,16 +4493,7 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc, > >>> > >>> void __page_frag_cache_drain(struct page *page, unsigned int count) > >>> { > >>> - VM_BUG_ON_PAGE(page_ref_count(page) == 0, page); > >>> - > >>> - if (page_ref_sub_and_test(page, count)) { > >>> - unsigned int order = compound_order(page); > >>> - > >>> - if (order == 0) > >>> - free_unref_page(page); > >>> - else > >>> - __free_pages_ok(page, order); > >>> - } > >>> + free_the_page(page, compound_order(page), count); > >>> } > >>> EXPORT_SYMBOL(__page_frag_cache_drain); > >>> > >>> @@ -4555,14 +4558,7 @@ void page_frag_free(void *addr) > >>> { > >>> struct page *page = virt_to_head_page(addr); > >>> > >>> - if (unlikely(put_page_testzero(page))) { > >>> - unsigned int order = compound_order(page); > >>> - > >>> - if (order == 0) > >>> - free_unref_page(page); > >>> - else > >>> - __free_pages_ok(page, order); > >>> - } > >>> + free_the_page(page, compound_order(page), 1); > >>> } > >>> EXPORT_SYMBOL(page_frag_free); > >>> > >>> > >> >