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

[Al Viro]

On Wed, Feb 12, 2020 at 10:45:06AM -0800, Linus Torvalds wrote:
> On Wed, Feb 12, 2020 at 7:01 AM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >
> > Fundamentally proc_flush_task is an optimization.  Just getting rid of
> > dentries earlier.  At least at one point it was an important
> > optimization because the old process dentries would just sit around
> > doing nothing for anyone.
> 
> I'm pretty sure it's still important. It's very easy to generate a
> _ton_ of dentries with /proc.
> 
> > I wonder if instead of invalidating specific dentries we could instead
> > fire wake up a shrinker and point it at one or more instances of proc.
> 
> It shouldn't be the dentries themselves that are a freeing problem.
> They're being RCU-free'd anyway because of lookup. It's the
> proc_mounts list that is the problem, isn't it?
> 
> So it's just fs_info that needs to be rcu-delayed because it contains
> that list. Or is there something else?

Large part of the headache is the possibility that some joker has
done something like mounting tmpfs on /proc/<pid>/map_files, or
binding /dev/null on top of /proc/<pid>/syscall, etc.

IOW, that d_invalidate() can very well have to grab namespace_sem.
And possibly do a full-blown fs shutdown of something NFS-mounted,
etc...