On Wed, Apr 09, 2025 at 04:25:10PM +0200, Sebastian Andrzej Siewior wrote: > On 2025-04-09 16:02:29 [+0200], Mateusz Guzik wrote: > > On Wed, Apr 09, 2025 at 03:14:44PM +0200, Sebastian Andrzej Siewior wrote: > > > One question: Do we need this lazy/ MNT_DETACH case? Couldn't we handle > > > them all via queue_rcu_work()? > > > If so, couldn't we have make deferred_free_mounts global and have two > > > release_list, say release_list and release_list_next_gp? The first one > > > will be used if queue_rcu_work() returns true, otherwise the second. > > > Then once defer_free_mounts() is done and release_list_next_gp not > > > empty, it would move release_list_next_gp -> release_list and invoke > > > queue_rcu_work(). > > > This would avoid the kmalloc, synchronize_rcu_expedited() and the > > > special-sauce. > > > > > > > To my understanding it was preferred for non-lazy unmount consumers to > > wait until the mntput before returning. > > > > That aside if I understood your approach it would de facto serialize all > > of these? > > > > As in with the posted patches you can have different worker threads > > progress in parallel as they all get a private list to iterate. > > > > With your proposal only one can do any work. > > > > One has to assume with sufficient mount/unmount traffic this can > > eventually get into trouble. > > Right, it would serialize them within the same worker thread. With one > worker for each put you would schedule multiple worker from the RCU > callback. Given the system_wq you will schedule them all on the CPU > which invokes the RCU callback. This kind of serializes it, too. > > The mntput() callback uses spinlock_t for locking and then it frees > resources. It does not look like it waits for something nor takes ages. > So it might not be needed to split each put into its own worker on a > different CPU… One busy bee might be enough ;) Unmounting can trigger very large number of mounts to be unmounted. If you're on a container heavy system or services that all propagate to each other in different mount namespaces mount propagation will generate a ton of umounts. So this cannot be underestimated. If a mount tree is wasted without MNT_DETACH it will pass UMOUNT_SYNC to umount_tree(). That'll cause MNT_SYNC_UMOUNT to be raised on all mounts during the unmount. If a concurrent path lookup calls legitimize_mnt() on such a mount and sees that MNT_SYNC_UMOUNT is set it will discount as it know that the concurrent unmounter hold the last reference and it __legitimize_mnt() can thus simply drop the reference count. The final mntput() will be done by the umounter. The synchronize_rcu() call in namespace_unlock() takes care that the last mntput() doesn't happen until path walking has dropped out of RCU mode. Without it it's possible that a non-MNT_DETACH umounter gets a spurious EBUSY error because a concurrent lazy path walk will suddenly put the last reference via mntput(). I'm unclear how that's handled in whatever it is you're proposing.
