Re: [PATCH v2] mount: dont execute propagate_umount() many times for same mounts

[Andrei Vagin <avagin-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>]

On Thu, Oct 06, 2016 at 02:46:30PM -0500, Eric W. Biederman wrote:
> Andrei Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org> writes:
> 
> > The reason of this optimization is that umount() can hold namespace_sem
> > for a long time, this semaphore is global, so it affects all users.
> > Recently Eric W. Biederman added a per mount namespace limit on the
> > number of mounts. The default number of mounts allowed per mount
> > namespace at 100,000. Currently this value is allowed to construct a tree
> > which requires hours to be umounted.
> 
> I am going to take a hard look at this as this problem sounds very
> unfortunate.  My memory of going through this code before strongly
> suggests that changing the last list_for_each_entry to
> list_for_each_entry_reverse is going to impact the correctness of this
> change.

I have read this code again and you are right, list_for_each_entry can't
be changed on list_for_each_entry_reverse here.

I tested these changes more carefully and find one more issue, so I am
going to send a new patch and would like to get your comments to it.

Thank you for your time.


> 
> The order of traversal is important if there are several things mounted
> one on the other that are all being unmounted.
> 
> Now perhaps your other changes have addressed that but I haven't looked
> closely enough to see that yet.
> 
> 
> > @@ -454,7 +473,7 @@ int propagate_umount(struct list_head *list)
> >  	list_for_each_entry_reverse(mnt, list, mnt_list)
> >  		mark_umount_candidates(mnt);
> >  
> > -	list_for_each_entry(mnt, list, mnt_list)
> > +	list_for_each_entry_reverse(mnt, list, mnt_list)
> >  		__propagate_umount(mnt);
> >  	return 0;
> >  }
> 
> Eric