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

[Neil Brown <neilb@suse.de>]

On Friday June 16, jblunck@suse.de wrote:
> On Mon, Jun 05, Neil Brown wrote:
> 
> > I understand that this is where problem is because the selected
> > dentries don't stay at the end of the list very long in some
> > circumstances. In particular, other filesystems' dentries get mixed
> > in. 
> 
> No. The problem is that the LRU list is too long and therefore unmounting
> seems to take ages.
> 

But I cannot see that the whole LRU list needs to be scanned during
unmount.
The only thing that does that is shrink_dcache_sb, which is used:
  in do_remount_sb
  in __invalidate_device
  in a few filesystems (autofs, coda, smbfs)
 and not when unmounting the filesystem (despite the comment).

(This is in 2.6.17-ec6-mm2).

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 I'm still puzzled as to why a long dcache LRU slows down
unmounting. 

Can you give more details?

Thanks,
NeilBrown