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...
