linux-ext4 - Re: [PATCH] ext4: group cache is added in ext4_mb_discard

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Message-ID: <20100406183158.GK23670@thunk.org>
Date:	Tue, 6 Apr 2010 14:31:58 -0400
From:	tytso@....edu
To:	jing zhang <zj.barak@...il.com>
Cc:	linux-ext4 <linux-ext4@...r.kernel.org>,
	Andreas Dilger <adilger@....com>,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] ext4: group cache is added in
 ext4_mb_discard_preallocations()

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.

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
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