From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 23 Jan 2008 00:39:50 +0100 From: Nick Piggin Subject: Re: [patch] #ifdef very expensive debug check in page fault path Message-ID: <20080122233950.GA29901@wotan.suse.de> References: <1200506488.32116.11.camel@cotte.boeblingen.de.ibm.com> <20080116234540.GB29823@wotan.suse.de> <20080116161021.c9a52c0f.akpm@linux-foundation.org> <479469A4.6090607@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org Return-Path: To: Hugh Dickins Cc: carsteno@de.ibm.com, Andrew Morton , Linux Memory Management List , mschwid2@linux.vnet.ibm.com, Holger Wolf , Linus Torvalds List-ID: On Tue, Jan 22, 2008 at 10:35:17PM +0000, Hugh Dickins wrote: > On Mon, 21 Jan 2008, Carsten Otte wrote: > > Hugh Dickins wrote: > > > > > > Well: that patch still gets my Nack, but I guess I'm too late. If > > > s390 pagetables are better protected than x86 ones, add an s390 ifdef? > > > > The alternative would be to just make > > #define pfn_valid(pfn) (1) > > on s390. That would also get _us_ rid of the check while others do benefit. We > > would trap access to mem_map beyond its limits because we don't have a kernel > > mapping there. For us, it would not silently corrupt things but crash proper. > > Whilst I quite like the sound of that, I wonder whether it's safe to > change s390's pfn_valid (rather surprisingly) for all its users. And > note that nobody but me has voiced any regret at the loss of the check. I did want to get rid of the test, but not in a "sneak it in before he notices" way. So I am disappointed it was merged before you replied. > My guess is we let it rest for now, and reconsider if a case comes up > later which would have got caught by the check (but the problem is that > such a case is much harder to identify than it was). The only cases I had imagined were repeatable things like a bug in pte manipulation somewhere, which will hopefully be caught with CONFIG_DEBUG_VM turned on. Are there many other cases where the test is useful? For hardware failures, I'd say not -- those just tend to waste developers time. -- 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