From: jing zhang Subject: Re: [PATCH] ext4: group cache is added in ext4_mb_discard_preallocations() Date: Wed, 7 Apr 2010 20:50:52 +0800 Message-ID: References: <20100406183158.GK23670@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: linux-ext4 , Andreas Dilger , Dave Kleikamp , "Aneesh Kumar K. V" To: tytso@mit.edu Return-path: Received: from mail-gx0-f217.google.com ([209.85.217.217]:36731 "EHLO mail-gx0-f217.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753503Ab0DGMvK (ORCPT ); Wed, 7 Apr 2010 08:51:10 -0400 Received: by gxk9 with SMTP id 9so629766gxk.8 for ; Wed, 07 Apr 2010 05:51:09 -0700 (PDT) In-Reply-To: <20100406183158.GK23670@thunk.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: 2010/4/7, tytso@mit.edu : > On Tue, Mar 30, 2010 at 08:36:17PM +0800, jing zhang wrote: >> --- linux-2.6.32/fs/ext4/mballoc.c 2009-12-03 11:51:22.000000000 +0800 >> +++ ext4_mm_leak/mballoc-13.c 2010-03-30 20:28:08.000000000 +0800 >> @@ -4183,12 +4183,20 @@ static int ext4_mb_discard_preallocation >> ext4_group_t i, ngroups = ext4_get_groups_count(sb); >> int ret; >> int freed = 0; >> + static ext4_group_t grp_cache = 0; > > This is a problem right there. Remember that there could be multiple > file systems mounted so a static variable is fundamentally flawed. > cool, the static in my patch is a fatal error. - zj > In fact, we could have a one filesystem which has more than 3 times > the number of groups as another file system. I'll leave it as an > exercise to a reader why your patch would be fundamentally flawed in > that case. > > The other thing to note is that this case only gets hit if the file > system is so full that we need to empty preallocations. So this means > hitting this case is rare, which raises two questions: (1) is it worth > it to optimize this case in the first place (is it really that > expensive to iterate over all the groups to discard the > preallocations); (2) can we test this case well? > > - Ted >