From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx159.postini.com [74.125.245.159]) by kanga.kvack.org (Postfix) with SMTP id AE39E6B0068 for ; Mon, 30 Jan 2012 12:54:40 -0500 (EST) Received: by ggnr5 with SMTP id r5so1640358ggn.14 for ; Mon, 30 Jan 2012 09:54:39 -0800 (PST) Date: Mon, 30 Jan 2012 09:54:34 -0800 From: Tejun Heo Subject: Re: [PATCH 1/3] percpu: use ZERO_SIZE_PTR / ZERO_OR_NULL_PTR Message-ID: <20120130175434.GG3355@google.com> References: <1327912654-8738-1-git-send-email-dmitry.antipov@linaro.org> <20120130171558.GB3355@google.com> <20120130174256.GF3355@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: Dmitry Antipov , Rusty Russell , linux-mm@kvack.org, linux-kernel@vger.kernel.org, patches@linaro.org, linaro-dev@lists.linaro.org Hello, Christoph. On Mon, Jan 30, 2012 at 11:52:23AM -0600, Christoph Lameter wrote: > We have two possibilities now: > > 1. We say that the value returned from the per cpu allocator is an opaque > value. > > This means that we have to remove the NULL check from the free > function. And audit the kernel code for all occurrences where > a per cpu pointer value of NULL is assumed to mean that no per > cpu allocation has occurred. No, NULL is never gonna be a valid return from any allocator including percpu. Percpu allocator doesn't and will never do so. > 2. We say that there are special values for the per cpu pointers (NULL, > ZERO_SIZE_PTR) > > Then we would have to guarantee that the per cpu allocator never > returns those values. > > Plus then the ZERO_SIZE_PTR patch will be fine. > > The danger exist of these values being passed as > parameters to functions that do not support them (per_cpu_ptr > etc). Those would need VM_BUG_ONs or some other checks to detect > potential problems. I'm saying we don't have this for ZERO_SIZE_PTR in any meaningful way at this point. If somebody wants to implement it properly, please feel free to, but simply applying ZERO_SIZE_PTR without other changes doesn't make any sense. Thanks. -- tejun -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org