linux-kernel - Re: [PATCH 0/5] Per-superblock shrinkers

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Message-ID: <20100515013005.GA31073@ZenIV.linux.org.uk>
Date:	Sat, 15 May 2010 02:30:05 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 0/5] Per-superblock shrinkers

On Fri, May 14, 2010 at 05:24:18PM +1000, Dave Chinner wrote:
> 
> This series reworks the filesystem shrinkers. We currently have a
> set of issues with the current filesystem shrinkers:
> 
> 	1. There is an dependency between dentry and inode cache
> 	   shrinking that is only implicitly defined by the order of
> 	   shrinker registration.
> 	2. The shrinkers need to walk the superblock list and pin
> 	   the superblock to avoid unmount races with the sb going
> 	   away.
> 	3. The dentry cache uses per-superblock LRUs and proportions
> 	   reclaim between all the superblocks which means we are
> 	   doing breadth based reclaim. This means we touch every
> 	   superblock for every shrinker call, and may only reclaim
> 	   a single dentry at a time from a given superblock.
> 	4. The inode cache has a global LRU, so it has different
> 	   reclaim patterns to the dentry cache, despite the fact
> 	   that the dentry cache is generally the only thing that
> 	   pins inodes in memory.
> 	5. Filesystems need to register their own shrinkers for
> 	   caches and can't co-ordinate them with the dentry and
> 	   inode cache shrinkers.

NAK in that form; sb refcounting and iterators had been reworked for .34,
so at least it needs rediff on top of that.  What's more, it's very
obviously broken wrt locking - you are unregistering a shrinker
from __put_super().  I.e. grab rwsem exclusively under a spinlock.

Essentially, you've turned dropping a _passive_ reference to superblock
(currently an operation safe in any context) into an operation allowed
only when no fs or vm locks are held by caller.  Not going to work...
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