linux-ext4 - Re: [PATCH] fs: allow for fs-specific objects to be pruned as part of pruning inodes

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Message-Id: <20130124005816.bce599de.akpm@linux-foundation.org>
Date:	Thu, 24 Jan 2013 00:58:16 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	gnehzuil.liu@...il.com, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: allow for fs-specific objects to be pruned as part
 of pruning inodes

On Thu, 24 Jan 2013 00:32:31 +1100 Dave Chinner <david@...morbit.com> wrote:

> Also, the superblock shrinker is designed around a direct 1:1:1
> dependency relationship between the superblock dentry, inode and "fs
> cache" objects. i.e. dentry pins inode pins fs cache object.  It is
> designed to keep a direct balance of the three caches by ensuring
> they get scanned in amounts directly proportional to the relative
> differences in their object counts.  That can't be done with
> separate shrinkers, hence the use of the superblock shrinker to
> define the dependent relationship between the caches.

I was staring at the code and at the 0e1fdafd9 changelog trying to work
out why prune_super() does its weird shrinker-in-a-shrinker thing.  And
failing.

IOW it needs a code comment, please.  Ideally one which explains *why*
"It is designed to keep a direct balance of the three caches...".  What
would go wrong if the fs were to just register its own shrinker in the
expected manner?

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