[Devel] Re: [Q] missing unused dentry in prune_dcache()?

[David Howells]

Vasily Averin <vvs at sw.ru> wrote:

> #   If prune_dcache finds a dentry that it cannot free, it leaves it where it
> #   is (at the tail of the list) and exits, on the assumption that some other
> #   thread will be removing that dentry soon.
> 
> However as far as I see this comment is not correct: when we cannot take
> s_umount rw_semaphore (for example because it was taken in do_remount) this
> dentry is already extracted from dentry_unused list and we do not add it into
> the list again.

You would seem to be correct.

> Therefore dentry will not be found by prune_dcache() and shrink_dcache_sb()
> and will leave in memory very long time until the partition will be
> unmounted.

And here too:-/

> Am I probably err?

Unfortunately not.  I wonder if remount should be getting a writelock on the
s_umount sem, but I don't see why not.  grab_super() also gets a writelock on
it, and so that could cause problems too.

shrink_dcache_for_umount_subtree() doesn't care because it doesn't scan the
dcache_unused list, but as you say, other things are affected.

> The patch adds this dentry into tail of the dentry_unused list.

I think that's reasonable.  I wonder if we can avoid removing it from the list
in the first place, but I suspect it's less optimal.

Acked-By: David Howells <dhowells at redhat.com>