Re: [patch 0/5] [PATCH,RFC] vfs: per-superblock unused dentries list (2nd version)

[David Chinner <dgc@sgi.com>]

On Mon, Jun 19, 2006 at 10:27:39AM +1000, Neil Brown wrote:
> On Monday June 19, dgc@sgi.com wrote:
> > 
> > > I can see that shrink_dcache_sb could take a long time and should be
> > > fixed, which should be as simple as replacing it with
> > > shrink_dcache_parent; shrink_dcache_anon.
> > 
> > But these are not guaranteed to reclaim all the dentries from a given
> > superblock. Yes, they move the dentries to the LRU, but other activity in the
> > system means that they may not get reclaimed during the subsequent calls
> > to prune_dcache() and hence they may live beyond the unmount....
> > 
> 
> My proposed patch earlier in this thread (I can post it again if you
> like) addresses exactly this issue.  Instead of moving dentries to the
> global LRU, it moves them to a private LRU, and the calls prune_dcache
> on that.  So there is no room for other activity to get in the way of
> prune_dcache doing what needs to be done.

Ok. That sounds like it would work.


> > > But I'm still puzzled as to why a long dcache LRU slows down
> > > unmounting. 
> > > 
> > > Can you give more details?
> > 
> > It's not the unmount that slows down - it's the fact that the dcache lock
> > is held for so long that rest of the system halts for time it takes
> > to run shrink_dcache_sb(). We've seen up to 50s to do a (touch fred; rm fred)
> > when the LRU has grown to several million dentries and shrink_dcache_sb()
> > is running. When this happens, it's not uncommon to see every CPU in the
> > machine spinning on the dcache_lock...
> 
> Definitely a problem.
> Maybe it was hoped that the call to cond_resched_lock(&dcache_lock)
> would avoid this, but apparently not.

The first pass over the LRU doesn't even do that....

> I still maintain that we should replace shrink_dcache_sb with calls to
> shrink_dcache_anon and shrink_dcache_parent.  That, together with my
> previous patch, should fix this problem quite cleanly.  If I send you
> a combined patch against the latest -mm can you test?

Ok. Send me the patch and I'll try to get some tests done on it...

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group