Re: [PATCH] Avoid useless inodes and dentries reclamation

[Tim Chen]

On Sat, 2013-08-31 at 19:00 +1000, Dave Chinner wrote:
> On Fri, Aug 30, 2013 at 09:21:34AM -0700, Tim Chen wrote:
> >
> > 
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> > diff --git a/fs/super.c b/fs/super.c
> > index 73d0952..4df1fab 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -112,9 +112,6 @@ static unsigned long super_cache_count(struct shrinker *shrink,
> >  
> >  	sb = container_of(shrink, struct super_block, s_shrink);
> >  
> > -	if (!grab_super_passive(sb))
> > -		return 0;
> > -
> 
> I think the function needs a comment explaining why we aren't
> grabbing the sb here, otherwise people are going to read the code
> and ask why it's different to the scanning callout.
> 
> >  	if (sb->s_op && sb->s_op->nr_cached_objects)
> >  		total_objects = sb->s_op->nr_cached_objects(sb,
> >  						 sc->nid);
> 

Yes, those comments are needed.
I also need to remove the corresponding
	drop_super(sb);

So probably something like:

---
diff --git a/fs/super.c b/fs/super.c
index 73d0952..7b5a6e5 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -112,9 +112,14 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 
 	sb = container_of(shrink, struct super_block, s_shrink);
 
-	if (!grab_super_passive(sb))
-		return 0;
-
+	/*
+	 * Don't call grab_super_passive as it is a potential 
+	 * scalability bottleneck. The counts could get updated 
+	 * between super_cache_count and super_cache_scan anyway.
+	 * Call to super_cache_count with shrinker_rwsem held
+	 * ensures the safety of call to list_lru_count_node() and 
+	 * s_op->nr_cached_objects().
+	 */
 	if (sb->s_op && sb->s_op->nr_cached_objects)
 		total_objects = sb->s_op->nr_cached_objects(sb,
 						 sc->nid);
@@ -125,7 +130,6 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 						 sc->nid);
 
 	total_objects = vfs_pressure_ratio(total_objects);
-	drop_super(sb);
 	return total_objects;
 }
 


> But seeing this triggered further thought on my part. Being called
> during unmount means that ->nr_cached_objects implementations need
> to be robust against unmount tearing down private filesystem
> structures.  Right now, grab_super_passive() protects us from that
> because it won't be able to get the sb->s_umount lock while
> generic_shutdown_super() is doing it's work.
> 
> IOWs, the superblock based shrinker operations are safe because the
> structures don't get torn down until after the shrinker is
> unregistered. That's not true for the structures that
> ->nr_cached_objects() use: ->put_super() tears them down before the
> shrinker is unregistered and only grab_super_passive() protects us
> from thay.
> 
> Let me have a bit more of a think about this - the solution may
> simply be unregistering the shrinker before we call ->kill_sb() so
> the shrinker can't get called while we are tearing down the fs.
> First, though, I need to go back and remind myself of why I put that
> after ->kill_sb() in the first place.  

Seems very reasonable as I haven't found a case where the shrinker 
is touched in ->kill_sb() yet. It looks like unregistering the
shrinker before ->kill_sb() should be okay.

> If we unregister the shrinker
> before ->kill_sb is called, then we can probably get rid of
> grab_super_passive() in both shrinker callouts because they will no
> longer need to handle running concurrently with ->kill_sb()....
> 

Thanks.

Tim

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