From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f200.google.com (mail-qk0-f200.google.com [209.85.220.200]) by kanga.kvack.org (Postfix) with ESMTP id 00A156B0253 for ; Tue, 11 Oct 2016 20:29:00 -0400 (EDT) Received: by mail-qk0-f200.google.com with SMTP id n189so24559326qke.0 for ; Tue, 11 Oct 2016 17:28:59 -0700 (PDT) Received: from sender153-mail.zoho.com (sender153-mail.zoho.com. [74.201.84.153]) by mx.google.com with ESMTPS id n7si2663564qkd.60.2016.10.11.17.28.59 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 11 Oct 2016 17:28:59 -0700 (PDT) Subject: Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when allocate a odd alignment area References: <20161011172228.GA30403@dhcp22.suse.cz> From: zijun_hu Message-ID: <7649b844-cfe6-abce-148e-1e2236e7d443@zoho.com> Date: Wed, 12 Oct 2016 08:28:17 +0800 MIME-Version: 1.0 In-Reply-To: <20161011172228.GA30403@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: zijun_hu@htc.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, tj@kernel.org, akpm@linux-foundation.org, cl@linux.com 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 don't know since there are too many sources and too many users and too many use cases. even if nobody, i can't be sure that it doesn't happens in the future it is worth since below reasons 1) if it is used in right ways, this patch have no impact; otherwise, it can alert user by warning message and correct the behavior. is it better that a warning message and correcting than resulting in many terrible error silently under a special case by change? it can make program more stronger. 2) does any alignment but 1 means a power of 2 alignment conventionally and implicitly? if not, is it better that adjusting both @align and @size uniformly based on the sufficient necessary condition than mixing supposing one part is right and correcting the other? i find that there is BUG_ON(!is_power_of_2(align)) statement in mm/vmalloc.c 3) this simple fix can make the function applicable in wider range, it hints the reader that the lowest requirement for alignment is a even number 4) for char a[10][10]; char (*p)[10]; if a user want to allocate a @size = 10 and @align = 10 memory block, should we reject the user's request? -- 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