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=-6.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 B8287C47247 for ; Mon, 4 May 2020 00:31:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9A45D20735 for ; Mon, 4 May 2020 00:31:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="bK4+bwjs" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726900AbgEDAbJ (ORCPT ); Sun, 3 May 2020 20:31:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35076 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1726683AbgEDAbI (ORCPT ); Sun, 3 May 2020 20:31:08 -0400 Received: from mail-qk1-x742.google.com (mail-qk1-x742.google.com [IPv6:2607:f8b0:4864:20::742]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9C844C061A0F for ; Sun, 3 May 2020 17:31:08 -0700 (PDT) Received: by mail-qk1-x742.google.com with SMTP id s9so12381129qkm.6 for ; Sun, 03 May 2020 17:31:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=Ta4uElbW6xoU05q9EisNotKHdMyGumUpwIk7d429KuY=; b=bK4+bwjsosYkVretPd6KcRggU2QP2snaoUIf8XtHrfU8iS5FxMrzm9JXspfXNRa3mq zWxEMdySKXcr1xJbtgmua5JYKkfZdBR69+G/2DYgIiyCkBS4LETSIV6qrhYP/N7PCMne O6wB/HGOVETLt+qigIeouS4qxNslp5SM/7Ppo= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Ta4uElbW6xoU05q9EisNotKHdMyGumUpwIk7d429KuY=; b=eW37rNyaDG7V9dYoul+Gkvoy6krG5daVWvf0a72gPUiPG3dsEUcAT56ZOJCl3li/Kh oFz5Inyu4ABfwB0JkhEq8ZqPeQUn1CL3wG2/wseFDsLm70tduqeyRVwZzsv8oKdmMNgG 1bwbf9zsGfrQW9ehj6WJ2IqPnVE0I8wSmJGn1wVKuvf/slkmpLqO92aIbxI1w2tJUu15 d6oJxX37LKASUaAe+T7JLxfL2uhcV07V6orFz1XH1m9t2ZaSVCEmKKESyJxlpDP9JHZH kCh6l/62fX80DSUDds8IQoPkCr+fVsXkzuLrGgNncbNI0lGN25fmdBmhtqB5UXGjkMNQ FNag== X-Gm-Message-State: AGi0PuZF5V4iKxIayLtYDsmaqDZuql4Wk5N3+5pVfXcY5mqLzWMxrGqB 0tZpEVCxfKilxnTc7K1WrNm4sA== X-Google-Smtp-Source: APiQypKMQfD3xwkZfKm7FXlyJStZaazEGR14S1Id8dbivLXWPQcn6uUzQHuucRtz3t1o8R1jlyjSsA== X-Received: by 2002:a37:804:: with SMTP id 4mr6151611qki.405.1588552267701; Sun, 03 May 2020 17:31:07 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id y185sm8705031qkd.29.2020.05.03.17.31.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 03 May 2020 17:31:07 -0700 (PDT) Date: Sun, 3 May 2020 20:31:06 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: "Uladzislau Rezki (Sony)" , LKML , linux-mm@kvack.org, Andrew Morton , "Theodore Y . Ts'o" , Matthew Wilcox , RCU , Oleksiy Avramchenko Subject: Re: [PATCH 20/24] rcu/tree: Make kvfree_rcu() tolerate any alignment Message-ID: <20200504003106.GC212435@google.com> References: <20200428205903.61704-1-urezki@gmail.com> <20200428205903.61704-21-urezki@gmail.com> <20200501230052.GG7560@paulmck-ThinkPad-P72> <20200504002437.GA212435@google.com> <20200504002947.GG2869@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200504002947.GG2869@paulmck-ThinkPad-P72> Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, May 03, 2020 at 05:29:47PM -0700, Paul E. McKenney wrote: > On Sun, May 03, 2020 at 08:24:37PM -0400, Joel Fernandes wrote: > > On Fri, May 01, 2020 at 04:00:52PM -0700, Paul E. McKenney wrote: > > > On Tue, Apr 28, 2020 at 10:58:59PM +0200, Uladzislau Rezki (Sony) wrote: > > > > From: "Joel Fernandes (Google)" > > > > > > > > Handle cases where the the object being kvfree_rcu()'d is not aligned by > > > > 2-byte boundaries. > > > > > > > > Signed-off-by: Uladzislau Rezki (Sony) > > > > Signed-off-by: Joel Fernandes (Google) > > > > --- > > > > kernel/rcu/tree.c | 9 ++++++--- > > > > 1 file changed, 6 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index 501cac02146d..649bad7ad0f0 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -2877,6 +2877,9 @@ struct kvfree_rcu_bulk_data { > > > > #define KVFREE_BULK_MAX_ENTR \ > > > > ((PAGE_SIZE - sizeof(struct kvfree_rcu_bulk_data)) / sizeof(void *)) > > > > > > > > +/* Encoding the offset of a fake rcu_head to indicate the head is a wrapper. */ > > > > +#define RCU_HEADLESS_KFREE BIT(31) > > > > > > Did I miss the check for freeing something larger than 2GB? Or is this > > > impossible, even on systems with many terabytes of physical memory? > > > Even if it is currently impossible, what prevents it from suddenly > > > becoming all too possible at some random point in the future? If you > > > think that this will never happen, please keep in mind that the first > > > time I heard "640K ought to be enough for anybody", it sounded eminently > > > reasonable to me. > > > > > > Besides... > > > > > > Isn't the offset in question the offset of an rcu_head struct within > > > the enclosing structure? If so, why not keep the current requirement > > > that this be at least 16-bit aligned, especially given that some work > > > is required to make that alignment less than pointer sized? Then you > > > can continue using bit 0. > > > > > > This alignment requirement is included in the RCU requirements > > > documentation and is enforced within the __call_rcu() function. > > > > > > So let's leave this at bit 0. > > > > This patch is needed only if we are growing the fake rcu_head. Since you > > mentioned in a previous patch in this series that you don't want to do that, > > and just rely on availability of the array of pointers or synchronize_rcu(), > > we can drop this patch. If we are not dropping that earlier patch, let us > > discuss more. > > Dropping it sounds very good to me! Cool ;-) Thanks, - Joel