The test case below fails on 5.17 (and a bunch of older kernels)
with unshare getting -ENOSPC, because the rate at which ipc_namespace
structures can be freed is limited by each item on the to-free list
waiting in synchronize_rcu.

Making kern_unmount use queue_rcu_work gets rid of that slowdown,
allowing a batch of vfsmount structures to be freed after each
RCU grace period has expired.

That, in turn, allows us to just get rid of the workqueue in
ipc/namespace.c completely.

With these two changes the test case reliably succeeds at
calling unshare a million times, even with max_ipc_namespaces
reduced to 1000 :)

#define _GNU_SOURCE
#include <sched.h>
#include <error.h>
#include <errno.h>
#include <stdlib.h>

int main()
{
	int i;

	for (i = 0; i < 1000000; i++) {
		if (unshare(CLONE_NEWIPC) < 0)
			error(EXIT_FAILURE, errno, "unshare");
	}
}


Rik van Riel (2):
  vfs: free vfsmount through rcu work from kern_unmount
  ipc: get rid of free_ipc_work workqueue

 fs/namespace.c                | 11 +++++++++--
 include/linux/ipc_namespace.h |  2 --
 include/linux/mount.h         |  2 ++
 ipc/namespace.c               | 21 +--------------------
 4 files changed, 12 insertions(+), 24 deletions(-)

-- 
2.34.1