Re: shrink_dcache_sb scalability problem.

["Bharata B Rao" <bharata.rao@gmail.com>]

On 4/15/06, Andrew Morton <akpm@osdl.org> wrote:
> "Bharata B Rao" <bharata.rao@gmail.com> wrote:
> >
> > > OT, I'm a bit curious about this:
> >  >
> >  >                 list_del_init(tmp);
> >  >                 spin_lock(&dentry->d_lock);
> >  >                 if (atomic_read(&dentry->d_count)) {
> >  >                         spin_unlock(&dentry->d_lock);
> >  >                         continue;
> >  >                 }
> >  >
> >  > So we rip the dentry off dcache_unused and just leave it floating about?
> >  > Dipankar, do you remember why that change was made, and why it's not a bug?
> >
> >  Due to lazy updating of the LRU list, there can be some dentries with non-zero
> >  ref counts on LRU list. This is one of the places where such dentries are
> >  removed from the LRU list. (Basically such dentries will be both on
> >  hash list and LRU
> >  list and here they get removed from the LRU list)
>
> OK.  But what guarantees that these live-but-detached dentries are
> appropriately destroyed before the unmount completes?

These are live dentries but not really detached, they are still attached to
the hash list. And yes I don't see shrink_dcache_sb holding the umount
because of these dentries. I assume dput of such dentries will happen
from different paths. But I am not sure if we could even end up in this
situation where we have landed up in shrink_dcache_sb from unmount path
and there are still some inuse dentries present. Need some clarification here.
>
> Or...  if these dentries will be freed by RCU callbacks potentially after
> the unmount, are we sure that they will always be in a state which will
> permit them to be freed?

When a dentry is queued for RCU freeing, there will be no references to
it from anywhere. It wouldn't be on hash list or on lru list. So I would think
only those dentries which are really freeable are queued for RCU freeing.

I see that shrink_dcache_sb is being called from the remount path
(do_remount_sb).
I couldn't understand why we do this. AFAICS we anyway don't modify the
mountpoint or the mount root during remount. Woudn't it be advantageous to
leave those dentries on LRU ?

Regards,
Bharata.