From 41c6838786c193bfeb2b2ff0c3d36c119217aeab Mon Sep 17 00:00:00 2001
From: Mark Johnston <markj@FreeBSD.org>
Date: Wed, 26 Aug 2020 14:31:35 +0000
Subject: [PATCH] vmem: Avoid allocating span tags when segments are never
 released.

vmem uses span tags to delimit imported segments, so that they can be
released if the segment becomes free in the future.  However, the
per-domain kernel KVA arenas never release resources, so the span tags
between imported ranges are unused when the ranges are contiguous.
Furthermore, such span tags prevent coalescing of free segments across
KVA_QUANTUM boundaries, resulting in internal fragmentation which
inhibits superpage promotion in the kernel map.

Stop allocating span tags in arenas that never release resources.  This
saves a small amount of memory and allows free segements to coalesce
across import boundaries.  This manifests as improved kernel superpage
usage during poudriere runs, which also helps to reduce physical memory
fragmentation by reducing the number of broken partially populated
reservations.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D24548
---
 sys/kern/subr_vmem.c | 63 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 50 insertions(+), 13 deletions(-)

diff --git a/sys/kern/subr_vmem.c b/sys/kern/subr_vmem.c
index 2c8da6634d7..b99b895233a 100644
--- a/sys/kern/subr_vmem.c
+++ b/sys/kern/subr_vmem.c
@@ -249,6 +249,18 @@ static struct vmem memguard_arena_storage;
 vmem_t *memguard_arena = &memguard_arena_storage;
 #endif
 
+static bool
+bt_isbusy(bt_t *bt)
+{
+	return (bt->bt_type == BT_TYPE_BUSY);
+}
+
+static bool
+bt_isfree(bt_t *bt)
+{
+	return (bt->bt_type == BT_TYPE_FREE);
+}
+
 /*
  * Fill the vmem's boundary tag cache.  We guarantee that boundary tag
  * allocation will not fail once bt_fill() passes.  To do so we cache
@@ -795,24 +807,48 @@ SYSINIT(vfs, SI_SUB_CONFIGURE, SI_ORDER_ANY, vmem_start_callout, NULL);
 static void
 vmem_add1(vmem_t *vm, vmem_addr_t addr, vmem_size_t size, int type)
 {
-	bt_t *btspan;
-	bt_t *btfree;
+	bt_t *btfree, *btprev, *btspan;
 
+	VMEM_ASSERT_LOCKED(vm);
 	MPASS(type == BT_TYPE_SPAN || type == BT_TYPE_SPAN_STATIC);
 	MPASS((size & vm->vm_quantum_mask) == 0);
 
-	btspan = bt_alloc(vm);
-	btspan->bt_type = type;
-	btspan->bt_start = addr;
-	btspan->bt_size = size;
-	bt_insseg_tail(vm, btspan);
+	if (vm->vm_releasefn == NULL) {
+		/*
+		 * The new segment will never be released, so see if it is
+		 * contiguous with respect to an existing segment.  In this case
+		 * a span tag is not needed, and it may be possible now or in
+		 * the future to coalesce the new segment with an existing free
+		 * segment.
+		 */
+		btprev = TAILQ_LAST(&vm->vm_seglist, vmem_seglist);
+		if ((!bt_isbusy(btprev) && !bt_isfree(btprev)) ||
+		    btprev->bt_start + btprev->bt_size != addr)
+			btprev = NULL;
+	} else {
+		btprev = NULL;
+	}
 
-	btfree = bt_alloc(vm);
-	btfree->bt_type = BT_TYPE_FREE;
-	btfree->bt_start = addr;
-	btfree->bt_size = size;
-	bt_insseg(vm, btfree, btspan);
-	bt_insfree(vm, btfree);
+	if (btprev == NULL || bt_isbusy(btprev)) {
+		if (btprev == NULL) {
+			btspan = bt_alloc(vm);
+			btspan->bt_type = type;
+			btspan->bt_start = addr;
+			btspan->bt_size = size;
+			bt_insseg_tail(vm, btspan);
+		}
+
+		btfree = bt_alloc(vm);
+		btfree->bt_type = BT_TYPE_FREE;
+		btfree->bt_start = addr;
+		btfree->bt_size = size;
+		bt_insseg_tail(vm, btfree);
+		bt_insfree(vm, btfree);
+	} else {
+		bt_remfree(vm, btprev);
+		btprev->bt_size += size;
+		bt_insfree(vm, btprev);
+	}
 
 	vm->vm_size += size;
 }
@@ -1147,6 +1183,7 @@ vmem_set_import(vmem_t *vm, vmem_import_t *importfn,
 {
 
 	VMEM_LOCK(vm);
+	KASSERT(vm->vm_size == 0, ("%s: arena is non-empty", __func__));
 	vm->vm_importfn = importfn;
 	vm->vm_releasefn = releasefn;
 	vm->vm_arg = arg;