From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f173.google.com (mail-wi0-f173.google.com [209.85.212.173]) by kanga.kvack.org (Postfix) with ESMTP id CC83D6B0032 for ; Thu, 7 May 2015 07:12:45 -0400 (EDT) Received: by wizk4 with SMTP id k4so238013467wiz.1 for ; Thu, 07 May 2015 04:12:45 -0700 (PDT) Received: from mail-wg0-x235.google.com (mail-wg0-x235.google.com. [2a00:1450:400c:c00::235]) by mx.google.com with ESMTPS id cs3si3282339wib.117.2015.05.07.04.12.44 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 07 May 2015 04:12:44 -0700 (PDT) Received: by wgic8 with SMTP id c8so13379855wgi.1 for ; Thu, 07 May 2015 04:12:43 -0700 (PDT) Date: Thu, 7 May 2015 13:12:39 +0200 From: Ingo Molnar Subject: Re: [PATCH RFC 01/15] uaccess: count pagefault_disable() levels in pagefault_disabled Message-ID: <20150507111239.GB15284@gmail.com> References: <1430934639-2131-1-git-send-email-dahi@linux.vnet.ibm.com> <1430934639-2131-2-git-send-email-dahi@linux.vnet.ibm.com> <20150507102254.GE23123@twins.programming.kicks-ass.net> <20150507125053.5d2e8f0a@thinkpad-w530> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150507125053.5d2e8f0a@thinkpad-w530> Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, mingo@redhat.com, yang.shi@windriver.com, bigeasy@linutronix.de, benh@kernel.crashing.org, paulus@samba.org, akpm@linux-foundation.org, heiko.carstens@de.ibm.com, schwidefsky@de.ibm.com, borntraeger@de.ibm.com, mst@redhat.com, tglx@linutronix.de, David.Laight@ACULAB.COM, hughd@google.com, hocko@suse.cz, ralf@linux-mips.org, herbert@gondor.apana.org.au, linux@arm.linux.org.uk, airlied@linux.ie, daniel.vetter@intel.com, linux-mm@kvack.org, linux-arch@vger.kernel.org * David Hildenbrand wrote: > > AFAICR we did this to avoid having to do both: > > > > preempt_disable(); > > pagefault_disable(); > > > > in a fair number of places -- just like this patch-set does, this is > > touching two cachelines where one would have been enough. > > > > Also, removing in_atomic() from fault handlers like you did > > significantly changes semantics for interrupts (soft, hard and NMI). > > > > So while I agree with most of these patches, I'm very hesitant on the > > above little detail. > > Just to make sure we have a common understanding (as written in my > cover letter): > > Your suggestion won't work with !CONFIG_PREEMPT > (!CONFIG_PREEMPT_COUNT). If there is no preempt counter, in_atomic() > won't work. So doing a preempt_disable() instead of a > pagefault_disable() is not going to work. (not sure how -RT handles > that - most probably with CONFIG_PREEMPT_COUNT being enabled, due to > atomic debug). > > That's why I dropped that check for a reason. So, what's the point of disabling the preempt counter? Looks like the much simpler (and faster) solution would be to eliminate CONFIG_PREEMPT_COUNT (i.e. make it always available), and use it for pagefault-disable. > This patchset is about decoupling both concept. (not ending up with > to mechanisms doing almost the same) So that's really backwards: just because we might not have a handy counter we introduce _another one_, and duplicate checks for it ;-) Why not keep a single counter, if indeed what we care about most in the pagefault_disable() case is atomicity? Thanks, Ingo -- 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