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

To: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH 0/5] Per-superblock shrinkers
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 17 May 2010 10:19:26 +1000
Cc: linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx
In-reply-to: <20100515013005.GA31073@xxxxxxxxxxxxxxxxxx>
References: <1273821863-29524-1-git-send-email-david@xxxxxxxxxxxxx> <20100515013005.GA31073@xxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
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.


Dave Chinner

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