Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756312AbXHTI7s (ORCPT ); Mon, 20 Aug 2007 04:59:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751479AbXHTI7j (ORCPT ); Mon, 20 Aug 2007 04:59:39 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:59781 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751877AbXHTI7i (ORCPT ); Mon, 20 Aug 2007 04:59:38 -0400 Subject: Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark? From: Peter Zijlstra To: Nigel Cunningham Cc: LKML In-Reply-To: <200708201838.50262.nigel@nigel.suspend2.net> References: <200708201138.50508.nigel@nigel.suspend2.net> <1187577830.6114.140.camel@twins> <200708201838.50262.nigel@nigel.suspend2.net> Content-Type: text/plain Date: Mon, 20 Aug 2007 10:59:36 +0200 Message-Id: <1187600376.6114.186.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.10.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2001 Lines: 47 On Mon, 2007-08-20 at 18:38 +1000, Nigel Cunningham wrote: > Hi. > > On Monday 20 August 2007 12:43:50 Peter Zijlstra wrote: > > On Mon, 2007-08-20 at 11:38 +1000, Nigel Cunningham wrote: > > > Hi all. > > > > > > In current git (and for a while now), an attempt to allocate memory with > > > GFP_ATOMIC will fail if we're below the low watermark level. The only way > to > > > access that memory that I can see (not that I've looked that hard) is to > have > > > PF_MEMALLOC set (ie from kswapd). I'm wondering if this behaviour is > correct. > > > Shouldn't GFP_ATOMIC allocations ignore watermarks too? How about > GFP_KERNEL? > > > > > > The following patch is a potential fix for GFP_ATOMIC. > > > > Sorry, no. > > > > GFP_ATOMIC must fail when below the watermark. GFP_KERNEL has __GFP_WAIT > > and hence can sleep and wait for reclaim so that should not be a problem > > (usually). > > > > GFP_ATOMIC may not access the reserves because the reserves are needed > > to get out of OOM deadlocks within the VM. Consider the fact that > > freeing memory needs memory - if there is no memory free, you cannot > > free memory and you're pretty much stuck. > > I guess, then, the question should be whether the watermark values are > appropriate. Do we need high order allocations watermarked if this is the > only purpose, particularly considering that whatever memory is allocated for > this purpose is essentially useless 99.9% of the time? Could you perhaps explain what you're trying to do? No matter what we do, GFP_ATOMIC will fail eventually, there is only so much one can do without blocking. As for higher order allocations, until we have a full online defrag solution those too can fail at any moment (even with __GFP_WAIT). - 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/