From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935427Ab1JEVwZ (ORCPT ); Wed, 5 Oct 2011 17:52:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:2172 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934884Ab1JEVwY (ORCPT ); Wed, 5 Oct 2011 17:52:24 -0400 Date: Wed, 5 Oct 2011 23:52:05 +0200 From: Johannes Weiner To: Dave Chinner Cc: Andrew Morton , Konstantin Khlebnikov , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] vmscan: fix initial shrinker size handling Message-ID: <20111005215205.GA16728@redhat.com> References: <20110822101721.19462.63082.stgit@zurg> <20110822232257.GT3162@dastard> <20110822163821.e746ab25.akpm@linux-foundation.org> <20110823000054.GW3162@dastard> <20110913183836.GA25998@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110913183836.GA25998@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 13, 2011 at 08:38:36PM +0200, Johannes Weiner wrote: > On Tue, Aug 23, 2011 at 10:00:54AM +1000, Dave Chinner wrote: > > On Mon, Aug 22, 2011 at 04:38:21PM -0700, Andrew Morton wrote: > > > On Tue, 23 Aug 2011 09:22:57 +1000 > > > Dave Chinner wrote: > > > > > > > On Mon, Aug 22, 2011 at 02:17:21PM +0300, Konstantin Khlebnikov wrote: > > > > > Shrinker function can returns -1, it means it cannot do anything without a risk of deadlock. > > > > > For example prune_super() do this if it cannot grab superblock refrence, even if nr_to_scan=0. > > > > > Currenly we interpret this like ULONG_MAX size shrinker, evaluate total_scan according this, > > > > > and next time this shrinker can get really big pressure. Let's skip such shrinkers instead. > > > > > > > > > > Also make total_scan signed, otherwise check (total_scan < 0) below never works. > > > > > > > > I've got a patch set I am going to post out today that makes this > > > > irrelevant. > > > > > > Well, how serious is the bug? If it's a non-issue then we can leave > > > the fix until 3.1. If it's a non-non-issue then we'd need a minimal > > > patch to fix up 3.1 and 3.0.x. > > > > I'm pretty sure it's a non-issue. I'm pretty sure all of the > > shrinkers return a count >= 0 rather than -1 when passed nr_to_scan > > == 0 (i.e. they skip the GFP_NOFS checking), so getting a max_pass > > of -1 isn't going to happen very often.... > > Except for the case which Konstantin laid out, grabbing the super > block reference. How likely is that? And why isn't once enough to > build up quite a high number? > > > And with total_scan being unsigned, the negative check is followed > > by a "if (total_scan > max_pass * 2)" check, which will catch > > numbers that would have gone negative anyway because max_pass won't > > be negative.... > > total_scan = nr; > max_pass = do_shrinker_shrink(shrinker, shrink, 0); > delta = (4 * nr_pages_scanned) / shrinker->seeks; > delta *= max_pass; > do_div(delta, lru_pages + 1); > total_scan += delta; > > max_pass, an unsigned long, is what the shrinker returned, so > ULONG_MAX. ULONG_MAX * 2 is ULONG_MAX - 1, still pretty big? > > Even for high values of delta (lots of pages scanned, few lru pages > left), it won't come nowhere near max_pass such that the product of > the two is a reasonable number again. Ping? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with SMTP id 58CD66B0253 for ; Wed, 5 Oct 2011 17:52:22 -0400 (EDT) Date: Wed, 5 Oct 2011 23:52:05 +0200 From: Johannes Weiner Subject: Re: [PATCH 1/2] vmscan: fix initial shrinker size handling Message-ID: <20111005215205.GA16728@redhat.com> References: <20110822101721.19462.63082.stgit@zurg> <20110822232257.GT3162@dastard> <20110822163821.e746ab25.akpm@linux-foundation.org> <20110823000054.GW3162@dastard> <20110913183836.GA25998@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110913183836.GA25998@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Dave Chinner Cc: Andrew Morton , Konstantin Khlebnikov , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Tue, Sep 13, 2011 at 08:38:36PM +0200, Johannes Weiner wrote: > On Tue, Aug 23, 2011 at 10:00:54AM +1000, Dave Chinner wrote: > > On Mon, Aug 22, 2011 at 04:38:21PM -0700, Andrew Morton wrote: > > > On Tue, 23 Aug 2011 09:22:57 +1000 > > > Dave Chinner wrote: > > > > > > > On Mon, Aug 22, 2011 at 02:17:21PM +0300, Konstantin Khlebnikov wrote: > > > > > Shrinker function can returns -1, it means it cannot do anything without a risk of deadlock. > > > > > For example prune_super() do this if it cannot grab superblock refrence, even if nr_to_scan=0. > > > > > Currenly we interpret this like ULONG_MAX size shrinker, evaluate total_scan according this, > > > > > and next time this shrinker can get really big pressure. Let's skip such shrinkers instead. > > > > > > > > > > Also make total_scan signed, otherwise check (total_scan < 0) below never works. > > > > > > > > I've got a patch set I am going to post out today that makes this > > > > irrelevant. > > > > > > Well, how serious is the bug? If it's a non-issue then we can leave > > > the fix until 3.1. If it's a non-non-issue then we'd need a minimal > > > patch to fix up 3.1 and 3.0.x. > > > > I'm pretty sure it's a non-issue. I'm pretty sure all of the > > shrinkers return a count >= 0 rather than -1 when passed nr_to_scan > > == 0 (i.e. they skip the GFP_NOFS checking), so getting a max_pass > > of -1 isn't going to happen very often.... > > Except for the case which Konstantin laid out, grabbing the super > block reference. How likely is that? And why isn't once enough to > build up quite a high number? > > > And with total_scan being unsigned, the negative check is followed > > by a "if (total_scan > max_pass * 2)" check, which will catch > > numbers that would have gone negative anyway because max_pass won't > > be negative.... > > total_scan = nr; > max_pass = do_shrinker_shrink(shrinker, shrink, 0); > delta = (4 * nr_pages_scanned) / shrinker->seeks; > delta *= max_pass; > do_div(delta, lru_pages + 1); > total_scan += delta; > > max_pass, an unsigned long, is what the shrinker returned, so > ULONG_MAX. ULONG_MAX * 2 is ULONG_MAX - 1, still pretty big? > > Even for high values of delta (lots of pages scanned, few lru pages > left), it won't come nowhere near max_pass such that the product of > the two is a reasonable number again. Ping? -- 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/ . 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