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

From: Dave Chinner
Date: Sun May 16 2010 - 20:19:46 EST

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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