From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx205.postini.com [74.125.245.205]) by kanga.kvack.org (Postfix) with SMTP id 5D1466B006E for ; Mon, 30 Jan 2012 12:58:56 -0500 (EST) Date: Mon, 30 Jan 2012 11:58:52 -0600 (CST) From: Christoph Lameter Subject: Re: [PATCH 1/3] percpu: use ZERO_SIZE_PTR / ZERO_OR_NULL_PTR In-Reply-To: <20120130175434.GG3355@google.com> Message-ID: References: <1327912654-8738-1-git-send-email-dmitry.antipov@linaro.org> <20120130171558.GB3355@google.com> <20120130174256.GF3355@google.com> <20120130175434.GG3355@google.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Tejun Heo Cc: Dmitry Antipov , Rusty Russell , linux-mm@kvack.org, linux-kernel@vger.kernel.org, patches@linaro.org, linaro-dev@lists.linaro.org On Mon, 30 Jan 2012, Tejun Heo wrote: > 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. How do you prevent the percpu allocator from returning NULL? I thought the per cpu offsets can wrap around? > > 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. We have no clean notion of how a percpu pointer needs to be handled. Both ways of handling things have drawbacks. -- 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