From 7ce3a31286088439cb854eea8225d41361322806 Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Tue, 9 Jun 2020 14:16:18 +0000
Subject: [PATCH] vm: rework swap_pager_status to execute in constant time

The lock-protected iteration is trivially avoidable.

This removes a serialisation point from Linux binaries (which end up calling
here from the sysinfo syscall).
---
 sys/vm/swap_pager.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/sys/vm/swap_pager.c b/sys/vm/swap_pager.c
index eae12d5a3953..74f8ed08d2cd 100644
--- a/sys/vm/swap_pager.c
+++ b/sys/vm/swap_pager.c
@@ -2541,16 +2541,10 @@ swapoff_all(void)
 void
 swap_pager_status(int *total, int *used)
 {
-	struct swdevt *sp;
 
-	*total = 0;
-	*used = 0;
-	mtx_lock(&sw_dev_mtx);
-	TAILQ_FOREACH(sp, &swtailq, sw_list) {
-		*total += sp->sw_nblks;
-		*used += sp->sw_used;
-	}
-	mtx_unlock(&sw_dev_mtx);
+	*total = swap_total;
+	*used = swap_total - swap_pager_avail -
+	    nswapdev * howmany(BBSIZE, PAGE_SIZE);
 }
 
 int