light: make MIN_SPACE runtime configurable

Kconfig

@@ -184,12 +184,16 @@ config MARS_MIN_SPACE_BASE
 	depends on MARS_MIN_SPACE
 	default 8
 	---help---
-	when this limit is exceeded, all write requests to /mars/
+	when this limit is exceeded, transaction logging to /mars/
 	will stop. This affects not only write IO to /dev/mars/*,
-	but also logfile transfers etc.
+	but also logfile transfers etc. As a consequence, writes
+	on the primary will directly go to the device, and the secondaries
+	will be outdated.
 	In order to retain full operations, you _need_ to implement
 	your own monitoring which _must_ warn you long before this
-	hard limit catches you.
+	hard limit catches you. If it already has caught you, it
+	will be too late: you have to recover by hand, e.g. by
+	starting a full sync.
 
 config MARS_MIN_SPACE_PERCENT
 	int "free space in /mars/ (hard limit in percent)"

sy_old/mars_light.c

@@ -159,7 +159,7 @@ EXPORT_SYMBOL_GPL(mars_mem_percent);
 #define COPY_PRIO MARS_PRIO_LOW
 
 #ifdef CONFIG_MARS_MIN_SPACE
-#define EXHAUSTED_LIMIT(max) ((max) / 100 * CONFIG_MARS_MIN_SPACE_PERCENT + CONFIG_MARS_MIN_SPACE_BASE * 1024 * 1024)
+#define EXHAUSTED_LIMIT(max) ((max) / 100 * CONFIG_MARS_MIN_SPACE_PERCENT + global_free_space * 1024 * 1024)
 #define EXHAUSTED(x,max) ((x) <= EXHAUSTED_LIMIT(max))
 #else
 #define EXHAUSTED_LIMIT(max) 0
@@ -3920,6 +3920,9 @@ static void __exit exit_light(void)
 int global_logrot_auto = CONFIG_MARS_LOGROT_AUTO;
 EXPORT_SYMBOL_GPL(global_logrot_auto);
 
+int global_free_space = CONFIG_MARS_MIN_SPACE_BASE;
+EXPORT_SYMBOL_GPL(global_free_space);
+
 static int __init init_light(void)
 {
 	int status = 0;

sy_old/mars_proc.c

@@ -190,6 +190,15 @@ ctl_table mars_table[] = {
 		.proc_handler	= &proc_dointvec,
 		.strategy       = &sysctl_intvec,
 	},
+	{
+		.ctl_name       = CTL_UNNUMBERED,
+		.procname	= "free_space_mb",
+		.data           = &global_free_space,
+		.maxlen         = sizeof(int),
+		.mode		= 0600,
+		.proc_handler	= &proc_dointvec,
+		.strategy       = &sysctl_intvec,
+	},
 #ifdef CONFIG_MARS_LOADAVG_LIMIT
 	{
 		.ctl_name       = CTL_UNNUMBERED,

sy_old/strategy.h

@@ -11,6 +11,7 @@
 #define MARS_PATH_MAX 256
 
 extern int global_logrot_auto;
+extern int global_free_space;
 
 extern char *my_id(void);