Further constrain the use of per-CPU caches for free pages.

In low memory conditions a significant number of pages may end up stuck
in the caches, and currently these caches cannot be reaped, leading to
spurious memory allocation failures and OOM kills.  So:

- Take into account the fact that we may cache up to two full buckets
  of pages per CPU, not just one.
- Increase the amount of RAM required per CPU to enable the caches.

This is a temporary measure until the page cache management policy is
improved.

PR:		241048
Reported and tested by:	Kevin Oberman <rkoberman@gmail.com>
Reviewed by:	alc, kib
Discussed with:	jeff
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D22040

sys/vm/vm_page.c

@@ -222,10 +222,12 @@ vm_page_init_cache_zones(void *dummy __unused)
 		vmd = VM_DOMAIN(domain);
 
 		/*
-		 * Don't allow the page caches to take up more than .25% of
-		 * memory.
+		 * Don't allow the page caches to take up more than .1875% of
+		 * memory.  A UMA bucket contains at most 256 free pages, and we
+		 * have two buckets per CPU per free pool.
 		 */
-		if (vmd->vmd_page_count / 400 < 256 * mp_ncpus * VM_NFREEPOOL)
+		if (vmd->vmd_page_count / 600 < 2 * 256 * mp_ncpus *
+		    VM_NFREEPOOL)
 			continue;
 		for (pool = 0; pool < VM_NFREEPOOL; pool++) {
 			pgcache = &vmd->vmd_pgcache[pool];