commit f8235976fc7430c9fe9110ce5b50e937faa210ee
Author: Mark Johnston <markjdb@gmail.com>
Date:   Wed Aug 12 13:08:49 2020 -0400

    uma: Respect uk_reserve in keg_drain().
    
    When we set a reserve of free items for a zone, we must take care to
    avoid reclamining them.  The two existing uma_zone_reserve() callers in
    the tree fail to set UMA_ZONE_NOFREE, which would have the desired
    effect.  Moreover, _NOFREE is a fairly large hammer.
    
    Modify keg_drain() to simply respect the reserved pool.  For now we take
    uk_reserve to be a per-NUMA domain quantity, since that's what existing
    code does.
    
    While here remove an always-false uk_freef == NULL check, and make sure
    that the keg_drain() KTR statement does not reference an uninitialized
    variable.

diff --git a/sys/vm/uma_core.c b/sys/vm/uma_core.c
index 9d0786c1d754..fe20cf415933 100644
--- a/sys/vm/uma_core.c
+++ b/sys/vm/uma_core.c
@@ -1404,44 +1404,70 @@ keg_free_slab(uma_keg_t keg, uma_slab_t slab, int start)
 	uma_total_dec(PAGE_SIZE * keg->uk_ppera);
 }
 
-/*
- * Frees pages from a keg back to the system.  This is done on demand from
- * the pageout daemon.
- *
- * Returns nothing.
- */
 static void
-keg_drain(uma_keg_t keg)
+keg_drain_domain(uma_keg_t keg, int domain)
 {
 	struct slabhead freeslabs;
 	uma_domain_t dom;
 	uma_slab_t slab, tmp;
-	int i, n;
+	int n, skip;
 
-	if (keg->uk_flags & UMA_ZONE_NOFREE || keg->uk_freef == NULL)
-		return;
+	dom = &keg->uk_domain[domain];
+	LIST_INIT(&freeslabs);
 
-	for (i = 0; i < vm_ndomains; i++) {
-		CTR4(KTR_UMA, "keg_drain %s(%p) domain %d free items: %u",
-		    keg->uk_name, keg, i, dom->ud_free_items);
-		dom = &keg->uk_domain[i];
-		LIST_INIT(&freeslabs);
+	CTR4(KTR_UMA, "keg_drain %s(%p) domain %d free items: %u",
+	    keg->uk_name, keg, i, dom->ud_free_items);
 
-		KEG_LOCK(keg, i);
+	KEG_LOCK(keg, domain);
+	if ((skip = keg->uk_reserve) != 0) {
 		if ((keg->uk_flags & UMA_ZFLAG_HASH) != 0) {
-			LIST_FOREACH(slab, &dom->ud_free_slab, us_link)
-				UMA_HASH_REMOVE(&keg->uk_hash, slab);
+			LIST_FOREACH(slab, &dom->ud_free_slab, us_link) {
+				if (skip > 0)
+					skip -= keg->uk_ppera;
+				else
+					UMA_HASH_REMOVE(&keg->uk_hash, slab);
+			}
+		}
+	} else {
+		LIST_FOREACH(slab, &dom->ud_free_slab, us_link)
+			UMA_HASH_REMOVE(&keg->uk_hash, slab);
+	}
+	LIST_SWAP(&freeslabs, &dom->ud_free_slab, uma_slab, us_link);
+	n = dom->ud_free_slabs;
+	if ((skip = keg->uk_reserve) != 0) {
+		LIST_FOREACH_SAFE(slab, &freeslabs, us_link, tmp) {
+			if (skip <= 0)
+				break;
+			LIST_REMOVE(slab, us_link);
+			LIST_INSERT_HEAD(&dom->ud_free_slab, slab, us_link);
+			skip -= keg->uk_ppera;
+			n--;
 		}
-		n = dom->ud_free_slabs;
-		LIST_SWAP(&freeslabs, &dom->ud_free_slab, uma_slab, us_link);
-		dom->ud_free_slabs = 0;
-		dom->ud_free_items -= n * keg->uk_ipers;
-		dom->ud_pages -= n * keg->uk_ppera;
-		KEG_UNLOCK(keg, i);
-
-		LIST_FOREACH_SAFE(slab, &freeslabs, us_link, tmp)
-			keg_free_slab(keg, slab, keg->uk_ipers);
 	}
+	dom->ud_free_slabs -= n;
+	dom->ud_free_items -= n * keg->uk_ipers;
+	dom->ud_pages -= n * keg->uk_ppera;
+	KEG_UNLOCK(keg, domain);
+
+	LIST_FOREACH_SAFE(slab, &freeslabs, us_link, tmp)
+		keg_free_slab(keg, slab, keg->uk_ipers);
+}
+
+/*
+ * Frees pages from a keg back to the system.  This is done on demand from
+ * the pageout daemon.
+ *
+ * Returns nothing.
+ */
+static void
+keg_drain(uma_keg_t keg)
+{
+	int i;
+
+	if ((keg->uk_flags & UMA_ZONE_NOFREE) != 0)
+		return;
+	for (i = 0; i < vm_ndomains; i++)
+		keg_drain_domain(keg, i);
 }
 
 static void