Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

Linus Torvalds [off-list ref] writes:

On Wed, Feb 12, 2020 at 1:48 PM Eric W. Biederman [off-list ref] wrote:

quoted

The good news is proc_flush_task isn't exactly called from process exit.
proc_flush_task is called during zombie clean up. AKA release_task.

Yeah, that at least avoids some of the nasty locking while dying debug problems.

But the one I was more worried about was actually the lock contention
issue with lots of processes. The lock is basically a single global
lock in many situations - yes, it's technically per-ns, but in a lot
of cases you really only have one namespace anyway.

And we've had problems with global locks in this area before, notably
the one you call out:

quoted

Further after proc_flush_task does it's thing the code goes
and does "write_lock_irq(&task_list_lock);"

Yeah, so it's not introducing a new issue, but it is potentially
making something we already know is bad even worse.

quoted

What would be downside of having a mutex for a list of proc superblocks?
A mutex that is taken for both reading and writing the list.

That's what the original patch actually was, and I was hoping we could
avoid that thing.

An rwsem would be possibly better, since most cases by far are likely
about reading.

And yes, I'm very aware of the task_list_lock, but it's literally why
I don't want to make a new one.

I'm _hoping_ we can some day come up with something better than
task_list_lock.

Yes.  I understand that.

I occassionally play with ideas, and converted all of proc to rcu
to help with situation but I haven't come up with anything clearly
better.


All of this is why I was really hoping we could have a change in
strategy and see if we can make the shrinker be able to better prune
proc inodes.



I think I have an alternate idea that could work.  Add some extra code
into proc_task_readdir, that would look for dentries that no longer
point to tasks and d_invalidate them.  With the same logic probably
being called from a few more places as well like proc_pid_readdir,
proc_task_lookup, and proc_pid_lookup.

We could even optimize it and have a process died flag we set in the
superblock.

That would would batch up the freeing work until the next time someone
reads from proc in a way that would create more dentries.  So it would
prevent dentries from reaped zombies from growing without bound.

Hmm.  Given the existence of proc_fill_cache it would really be a good
idea if readdir and lookup performed some of the freeing work as well.
As on readdir we always populate the dcache for all of the directory
entries.

I am booked solid for the next little while but if no one beats me to it
I will try and code something like that up where at least readdir
looks for and invalidates stale dentries.

Eric