Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755199AbZDVRML (ORCPT ); Wed, 22 Apr 2009 13:12:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752740AbZDVRL4 (ORCPT ); Wed, 22 Apr 2009 13:11:56 -0400 Received: from gir.skynet.ie ([193.1.99.77]:43838 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751543AbZDVRLz (ORCPT ); Wed, 22 Apr 2009 13:11:55 -0400 Date: Wed, 22 Apr 2009 18:11:51 +0100 From: Mel Gorman To: Dave Hansen Cc: Linux Memory Management List , KOSAKI Motohiro , Christoph Lameter , Nick Piggin , Linux Kernel Mailing List , Lin Ming , Zhang Yanmin , Peter Zijlstra , Pekka Enberg , Andrew Morton Subject: Re: [PATCH 02/22] Do not sanity check order in the fast path Message-ID: <20090422171151.GF15367@csn.ul.ie> References: <1240408407-21848-1-git-send-email-mel@csn.ul.ie> <1240408407-21848-3-git-send-email-mel@csn.ul.ie> <1240416791.10627.78.camel@nimitz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1240416791.10627.78.camel@nimitz> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3347 Lines: 81 On Wed, Apr 22, 2009 at 09:13:11AM -0700, Dave Hansen wrote: > On Wed, 2009-04-22 at 14:53 +0100, Mel Gorman wrote: > > No user of the allocator API should be passing in an order >= MAX_ORDER > > but we check for it on each and every allocation. Delete this check and > > make it a VM_BUG_ON check further down the call path. > > Should we get the check re-added to some of the upper-level functions, > then? Perhaps __get_free_pages() or things like alloc_pages_exact()? > I don't think so, no. It just moves the source of the text bloat and for the few callers that are asking for something that will never succeed. > I'm selfishly thinking of what I did in profile_init(). Can I slab > alloc it? Nope. Page allocator? Nope. Oh, well, try vmalloc(): > > prof_buffer = kzalloc(buffer_bytes, GFP_KERNEL); > if (prof_buffer) > return 0; > > prof_buffer = alloc_pages_exact(buffer_bytes, GFP_KERNEL|__GFP_ZERO); > if (prof_buffer) > return 0; > > prof_buffer = vmalloc(buffer_bytes); > if (prof_buffer) > return 0; > > free_cpumask_var(prof_cpu_mask); > return -ENOMEM; > Can this ever actually be asking for an order larger than MAX_ORDER though? If so, you're condemning it to always behave poorly. > Same thing in __kmalloc_section_memmap(): > > page = alloc_pages(GFP_KERNEL|__GFP_NOWARN, get_order(memmap_size)); > if (page) > goto got_map_page; > > ret = vmalloc(memmap_size); > if (ret) > goto got_map_ptr; > If I'm reading that right, the order will never be a stupid order. It can fail for higher orders in which case it falls back to vmalloc() . For example, to hit that limit, the section size for a 4K kernel, maximum usable order of 10, the section size would need to be 256MB (assuming struct page size of 64 bytes). I don't think it's ever that size and if so, it'll always be sub-optimal which is a poor choice to make. > I depend on the allocator to tell me when I've fed it too high of an > order. If we really need this, perhaps we should do an audit and then > add a WARN_ON() for a few releases to catch the stragglers. > I consider it buggy to ask for something so large that you always end up with the worst option - vmalloc(). How about leaving it as a VM_BUG_ON to get as many reports as possible on who is depending on this odd behaviour? If there are users with good reasons, then we could convert this to WARN_ON to fix up the callers. I suspect that the allocator can already cope with recieving a stupid order silently but slowly. It should go all the way to the bottom and just never find anything useful and return NULL. zone_watermark_ok is the most dangerous looking part but even it should never get to MAX_ORDER because it should always find there are not enough free pages and return before it overruns. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/