From b888f8da8e1e89871371706004b82d2a0f2d74ae Mon Sep 17 00:00:00 2001
From: des <des@ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f>
Date: Tue, 4 Sep 2012 19:14:57 +0000
Subject: [PATCH] MFH (r239255, r239730): increase maxswzone on i386 and remove
 it on amd64.

git-svn-id: svn://svn.freebsd.org/base/stable/9@240097 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
---
 sys/amd64/include/param.h |  8 --------
 sys/boot/common/loader.8  | 31 ++++++++++++++++++++-----------
 sys/i386/include/param.h  |  8 +++++++-
 3 files changed, 27 insertions(+), 20 deletions(-)

diff --git a/sys/amd64/include/param.h b/sys/amd64/include/param.h
index 6dbeb4035..9ddcf685b 100644
--- a/sys/amd64/include/param.h
+++ b/sys/amd64/include/param.h
@@ -122,14 +122,6 @@
 #endif
 #define	KSTACK_GUARD_PAGES 1	/* pages of kstack guard; 0 disables */
 
-/*
- * Ceiling on amount of swblock kva space, can be changed via
- * the kern.maxswzone /boot/loader.conf variable.
- */
-#ifndef VM_SWZONE_SIZE_MAX
-#define	VM_SWZONE_SIZE_MAX	(32 * 1024 * 1024)
-#endif
-
 /*
  * Mach derived conversion macros
  */
diff --git a/sys/boot/common/loader.8 b/sys/boot/common/loader.8
index cbb989132..87f86d5ab 100644
--- a/sys/boot/common/loader.8
+++ b/sys/boot/common/loader.8
@@ -613,17 +613,26 @@ Not all architectures use such buffers; see
 for details.
 .It Va kern.maxswzone
 Limits the amount of KVM to be used to hold swap
-meta information, which directly governs the
-maximum amount of swap the system can support.
-This value is specified in bytes of KVA space
-and defaults to 32MBytes on i386 and amd64.
-Care should be taken
-to not reduce this value such that the actual
-amount of configured swap exceeds 1/2 the
-kernel-supported swap.
-The default of 32MB allows
-the kernel to support a maximum of ~7GB of swap.
-Only change
+metadata, which directly governs the
+maximum amount of swap the system can support,
+at the rate of approximately 200 MB of swap space
+per 1 MB of metadata.
+This value is specified in bytes of KVA space.
+If no value is provided, the system allocates
+enough memory to handle an amount of swap
+that corresponds to eight times the amount of
+physical memory present in the system.
+.Pp
+Note that swap metadata can be fragmented,
+which means that the system can run out of
+space before it reaches the theoretical limit.
+Therefore, care should be taken to not configure
+more swap than approximately half of the
+theoretical maximum.
+.Pp
+Running out of space for swap metadata can leave
+the system in an unrecoverable state.
+Therefore, you should only change
 this parameter if you need to greatly extend the
 KVM reservation for other resources such as the
 buffer cache or
diff --git a/sys/i386/include/param.h b/sys/i386/include/param.h
index 6a0495c4c..b494f3509 100644
--- a/sys/i386/include/param.h
+++ b/sys/i386/include/param.h
@@ -125,9 +125,15 @@
 /*
  * Ceiling on amount of swblock kva space, can be changed via
  * the kern.maxswzone /boot/loader.conf variable.
+ *
+ * 276 is sizeof(struct swblock), but we do not always have a definition
+ * in scope for struct swblock, so we have to hardcode it.  Each struct
+ * swblock holds metadata for 32 pages, so in theory, this is enough for
+ * 16 GB of swap.  In practice, however, the usable amount is considerably
+ * lower due to fragmentation.
  */
 #ifndef VM_SWZONE_SIZE_MAX
-#define VM_SWZONE_SIZE_MAX	(32 * 1024 * 1024)
+#define VM_SWZONE_SIZE_MAX	(276 * 128 * 1024)
 #endif
 
 /*
-- 
2.45.0