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 0EB8CC7618A for ; Mon, 20 Mar 2023 15:22:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232719AbjCTPWY (ORCPT ); Mon, 20 Mar 2023 11:22:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58932 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232607AbjCTPVp (ORCPT ); Mon, 20 Mar 2023 11:21:45 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0D39118AB4; Mon, 20 Mar 2023 08:15:33 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id A1FC221AAD; Mon, 20 Mar 2023 15:14:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1679325290; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=uLZs4PB9+G025pPf5iXZq69Oo8p6DwoTDJ9FIB3DakA=; b=dyfo88zYl6Yg5zwB//QpN0nOKP+K1umn0wM32BDTTjYeqCJ5p9Lw2jOa3SK4qiWcMcrwlj Fy6X5H1yeX38+Va2dUprUcfbeFAEtq8A9KSvKaJDEgyOlLaxVyisCGZWnvVy2frIv5Mm+q gDYPobFxbx1LkmjqgawM9lxRqlfRiho= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1679325290; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=uLZs4PB9+G025pPf5iXZq69Oo8p6DwoTDJ9FIB3DakA=; b=+sFPLg9Lj4GgT1NOz/fE/eXtrZorlXjvebBjXnJtiJxzsB8z3hapyRq41TxCT4rp0QOXBH spDHqXlbU7W1nbCQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 88EC513A00; Mon, 20 Mar 2023 15:14:50 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 6mHdIGp4GGTcPgAAMHmgww (envelope-from ); Mon, 20 Mar 2023 15:14:50 +0000 Message-ID: <7b2a7b7b-0ebc-1f03-5f1b-ac598fc950dc@suse.cz> Date: Mon, 20 Mar 2023 16:14:50 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Subject: Re: [net PATCH 1/2] mm: Use fixed constant in page_frag_alloc instead of size + 1 Content-Language: en-US From: Vlastimil Babka To: Alexander Duyck , netdev@vger.kernel.org, davem@davemloft.net Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, jannh@google.com References: <20190215223741.16881.84864.stgit@localhost.localdomain> <20190215224412.16881.89296.stgit@localhost.localdomain> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2/17/23 10:30, Vlastimil Babka wrote: > On 2/15/19 23:44, Alexander Duyck wrote: >> From: Alexander Duyck >> >> This patch replaces the size + 1 value introduced with the recent fix for 1 >> byte allocs with a constant value. >> >> The idea here is to reduce code overhead as the previous logic would have >> to read size into a register, then increment it, and write it back to >> whatever field was being used. By using a constant we can avoid those >> memory reads and arithmetic operations in favor of just encoding the >> maximum value into the operation itself. >> >> Fixes: 2c2ade81741c ("mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs") >> Signed-off-by: Alexander Duyck >> --- >> mm/page_alloc.c | 8 ++++---- >> 1 file changed, 4 insertions(+), 4 deletions(-) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index ebb35e4d0d90..37ed14ad0b59 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -4857,11 +4857,11 @@ void *page_frag_alloc(struct page_frag_cache *nc, >> /* Even if we own the page, we do not use atomic_set(). >> * This would break get_page_unless_zero() users. >> */ >> - page_ref_add(page, size); >> + page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE); > > But this value can be theoretically too low when PAGE_SIZE > > PAGE_FRAG_CACHE_MAX_SIZE? Such as on architectures with 64kB page size, > while PAGE_FRAG_CACHE_MAX_SIZE is 32kB? Nevermind, PAGE_FRAG_CACHE_MAX_SIZE would be 64kB because #define PAGE_FRAG_CACHE_MAX_SIZE __ALIGN_MASK(32768, ~PAGE_MASK) So all is fine, sorry for the noise.