From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id 8D05C6B025E for ; Wed, 12 Oct 2016 04:25:41 -0400 (EDT) Received: by mail-wm0-f72.google.com with SMTP id f193so5199544wmg.2 for ; Wed, 12 Oct 2016 01:25:41 -0700 (PDT) Received: from mail-wm0-f65.google.com (mail-wm0-f65.google.com. [74.125.82.65]) by mx.google.com with ESMTPS id ql1si9017517wjc.85.2016.10.12.01.25.40 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 12 Oct 2016 01:25:40 -0700 (PDT) Received: by mail-wm0-f65.google.com with SMTP id 123so1213737wmb.2 for ; Wed, 12 Oct 2016 01:25:40 -0700 (PDT) Date: Wed, 12 Oct 2016 10:25:38 +0200 From: Michal Hocko Subject: Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when allocate a odd alignment area Message-ID: <20161012082538.GC17128@dhcp22.suse.cz> References: <20161011172228.GA30403@dhcp22.suse.cz> <7649b844-cfe6-abce-148e-1e2236e7d443@zoho.com> <20161012065332.GA9504@dhcp22.suse.cz> <57FDE531.7060003@zoho.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57FDE531.7060003@zoho.com> Sender: owner-linux-mm@kvack.org List-ID: To: zijun_hu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, zijun_hu@htc.com, tj@kernel.org, akpm@linux-foundation.org, cl@linux.com On Wed 12-10-16 15:24:33, zijun_hu wrote: > On 10/12/2016 02:53 PM, Michal Hocko wrote: > > On Wed 12-10-16 08:28:17, zijun_hu wrote: > >> On 2016/10/12 1:22, Michal Hocko wrote: > >>> On Tue 11-10-16 21:24:50, zijun_hu wrote: > >>>> From: zijun_hu > >>>> > >>>> the LSB of a chunk->map element is used for free/in-use flag of a area > >>>> and the other bits for offset, the sufficient and necessary condition of > >>>> this usage is that both size and alignment of a area must be even numbers > >>>> however, pcpu_alloc() doesn't force its @align parameter a even number > >>>> explicitly, so a odd @align maybe causes a series of errors, see below > >>>> example for concrete descriptions. > >>> > >>> Is or was there any user who would use a different than even (or power of 2) > >>> alighment? If not is this really worth handling? > >>> > >> > >> it seems only a power of 2 alignment except 1 can make sure it work very well, > >> that is a strict limit, maybe this more strict limit should be checked > > > > I fail to see how any other alignment would actually make any sense > > what so ever. Look, I am not a maintainer of this code but adding a new > > code to catch something that doesn't make any sense sounds dubious at > > best to me. > > > > I could understand this patch if you see a problem and want to prevent > > it from repeating bug doing these kind of changes just in case sounds > > like a bad idea. > > > > thanks for your reply > > should we have a generic discussion whether such patches which considers > many boundary or rare conditions are necessary. In general, I believe that kernel internal interfaces which have no userspace exposure shouldn't be cluttered with sanity checks. > i found the following code segments in mm/vmalloc.c > static struct vmap_area *alloc_vmap_area(unsigned long size, > unsigned long align, > unsigned long vstart, unsigned long vend, > int node, gfp_t gfp_mask) > { > ... > > BUG_ON(!size); > BUG_ON(offset_in_page(size)); > BUG_ON(!is_power_of_2(align)); See a recent Linus rant about BUG_ONs. These BUG_ONs are quite old and from a quick look they are even unnecessary. So rather than adding more of those, I think removing those that are not needed is much more preferred. > should we make below declarations as conventions > 1) when we say 'alignment', it means align to a power of 2 value > for example, aligning value @v to @b implicit @v is power of 2 > , align 10 to 4 is 12 alignment other than power-of-two makes only very limited sense to me. > 2) when we say 'round value @v up/down to boundary @b', it means the > result is a times of @b, it don't requires @b is a power of 2 -- Michal Hocko SUSE Labs -- 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