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

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Date:	Mon, 17 May 2010 10:19:26 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Al Viro <viro@...IV.linux.org.uk>
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 Sat, May 15, 2010 at 02:30:05AM +0100, Al Viro wrote:
> 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.

The tree I based this on was 2.6.34-rc7 - is there new code in a
-next branch somewhere?

> 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...

Yeah, I picked that up after I posted it. My bad - I'll look into how
I can rework that for the next iteration.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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