From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f49.google.com (mail-wg0-f49.google.com [74.125.82.49]) by kanga.kvack.org (Postfix) with ESMTP id 899666B0069 for ; Mon, 27 Oct 2014 10:40:18 -0400 (EDT) Received: by mail-wg0-f49.google.com with SMTP id x13so2095979wgg.8 for ; Mon, 27 Oct 2014 07:40:16 -0700 (PDT) Received: from mailapp01.imgtec.com (mailapp01.imgtec.com. [195.59.15.196]) by mx.google.com with ESMTP id ez12si9226037wid.10.2014.10.27.07.40.15 for ; Mon, 27 Oct 2014 07:40:16 -0700 (PDT) Message-ID: <544E58E3.9040100@imgtec.com> Date: Mon, 27 Oct 2014 14:38:27 +0000 From: Zubair Lutfullah Kakakhel MIME-Version: 1.0 Subject: Re: [RFC] mm: memblock: change default cnt for regions from 1 to 0 References: <1414083413-61756-1-git-send-email-Zubair.Kakakhel@imgtec.com> <20141023121840.f88439912f23a3c2a01eb54f@linux-foundation.org> <20141027141730.GL4436@htj.dyndns.org> In-Reply-To: <20141027141730.GL4436@htj.dyndns.org> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Tejun Heo , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Tang Chen , Yinghai Lu On 27/10/14 14:17, Tejun Heo wrote: > Hello, > > On Thu, Oct 23, 2014 at 12:18:40PM -0700, Andrew Morton wrote: >> On Thu, 23 Oct 2014 17:56:53 +0100 Zubair Lutfullah Kakakhel wrote: >> >>> The default region counts are set to 1 with a comment saying empty >>> dummy entry. >>> >>> If this is a dummy entry, should this be changed to 0? > > My memory is hazy now but I'm pretty sure there's a bunch of stuff > assuming that the array is never empty. > >>> We have faced this in mips/kernel/setup.c arch_mem_init. >>> >>> cma uses memblock. But even with cma disabled. >>> The for_each_memblock(reserved, reg) goes inside the loop. >>> Even without any reserved regions. > > Does that matter? It's a zero-length reservation. > >>> Traced it to the following, when the macro >>> for_each_memblock(memblock_type, region) is used. >>> >>> It expands to add the cnt variable. >>> >>> for (region = memblock.memblock_type.regions; \ >>> region < (memblock.memblock_type.regions + memblock.memblock_type.cnt); \ >>> region++) >>> >>> In the corner case, that there are no reserved regions. >>> Due to the default 1 value of cnt. >>> The loop under for_each_memblock still runs once. >>> >>> Even when there is no reserved region. >>> >>> Is this by design? or unintentional? > > It's by design. > >>> It might be that this loop runs an extra time every instance out there? > > The first actual entry replaces the dummy one and the last removal > makes the entry dummy again, so the dummy one exists iff that's the > only entry. I don't recall the exact details right now but the choice > was an intentional one. > > Thanks. > Thank-you for clarifying. Regards ZubairLK -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org