ipc: recompute msgmni on memory add / remove

Introduce the registration of a callback routine that recomputes msg_ctlmni
upon memory add / remove.

A single notifier block is registered in the hotplug memory chain for all the
ipc namespaces.

Since the ipc namespaces are not linked together, they have their own
notification chain: one notifier_block is defined per ipc namespace.

Each time an ipc namespace is created (removed) it registers (unregisters) its
notifier block in (from) the ipcns chain.  The callback routine registered in
the memory chain invokes the ipcns notifier chain with the IPCNS_LOWMEM event.
 Each callback routine registered in the ipcns namespace, in turn, recomputes
msgmni for the owning namespace.

Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Mingming Cao <cmm@us.ibm.com>
Cc: Pierre Peiffer <pierre.peiffer@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

6 files changed, 120 insertions, 2 deletions

diff --git a/ipc/Makefile b/ipc/Makefile
index 5fc5e33ea04..388e4d259f0 100644
--- a/ipc/Makefile
+++ b/ipc/Makefile
@@ -3,7 +3,8 @@
 #
 
 obj-$(CONFIG_SYSVIPC_COMPAT) += compat.o
-obj-$(CONFIG_SYSVIPC) += util.o msgutil.o msg.o sem.o shm.o
+obj_mem-$(CONFIG_MEMORY_HOTPLUG) += ipcns_notifier.o
+obj-$(CONFIG_SYSVIPC) += util.o msgutil.o msg.o sem.o shm.o $(obj_mem-y)
 obj-$(CONFIG_SYSVIPC_SYSCTL) += ipc_sysctl.o
 obj_mq-$(CONFIG_COMPAT) += compat_mq.o
 obj-$(CONFIG_POSIX_MQUEUE) += mqueue.o msgutil.o $(obj_mq-y)
diff --git a/ipc/ipcns_notifier.c b/ipc/ipcns_notifier.c
new file mode 100644
index 00000000000..0786af6ce3e
--- /dev/null
+++ b/ipc/ipcns_notifier.c
@@ -0,0 +1,71 @@
+/*
+ * linux/ipc/ipcns_notifier.c
+ * Copyright (C) 2007 BULL SA. Nadia Derbey
+ *
+ * Notification mechanism for ipc namespaces:
+ * The callback routine registered in the memory chain invokes the ipcns
+ * notifier chain with the IPCNS_MEMCHANGED event.
+ * Each callback routine registered in the ipcns namespace recomputes msgmni
+ * for the owning namespace.
+ */
+
+#include <linux/msg.h>
+#include <linux/rcupdate.h>
+#include <linux/notifier.h>
+#include <linux/nsproxy.h>
+#include <linux/ipc_namespace.h>
+
+#include "util.h"
+
+
+
+static BLOCKING_NOTIFIER_HEAD(ipcns_chain);
+
+
+static int ipcns_callback(struct notifier_block *self,
+				unsigned long action, void *arg)
+{
+	struct ipc_namespace *ns;
+
+	switch (action) {
+	case IPCNS_MEMCHANGED:   /* amount of lowmem has changed */
+		/*
+		 * It's time to recompute msgmni
+		 */
+		ns = container_of(self, struct ipc_namespace, ipcns_nb);
+		/*
+		 * No need to get a reference on the ns: the 1st job of
+		 * free_ipc_ns() is to unregister the callback routine.
+		 * blocking_notifier_chain_unregister takes the wr lock to do
+		 * it.
+		 * When this callback routine is called the rd lock is held by
+		 * blocking_notifier_call_chain.
+		 * So the ipc ns cannot be freed while we are here.
+		 */
+		recompute_msgmni(ns);
+		break;
+	default:
+		break;
+	}
+
+	return NOTIFY_OK;
+}
+
+int register_ipcns_notifier(struct ipc_namespace *ns)
+{
+	memset(&ns->ipcns_nb, 0, sizeof(ns->ipcns_nb));
+	ns->ipcns_nb.notifier_call = ipcns_callback;
+	ns->ipcns_nb.priority = IPCNS_CALLBACK_PRI;
+	return blocking_notifier_chain_register(&ipcns_chain, &ns->ipcns_nb);
+}
+
+int unregister_ipcns_notifier(struct ipc_namespace *ns)
+{
+	return blocking_notifier_chain_unregister(&ipcns_chain,
+						&ns->ipcns_nb);
+}
+
+int ipcns_notify(unsigned long val)
+{
+	return blocking_notifier_call_chain(&ipcns_chain, val, NULL);
+}
diff --git a/ipc/msg.c b/ipc/msg.c
index be8449d48a8..7d9b0694c74 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -84,7 +84,7 @@ static int sysvipc_msg_proc_show(struct seq_file *s, void *it);
  * Also take into account the number of nsproxies created so far.
  * This should be done staying within the (MSGMNI , IPCMNI/nr_ipc_ns) range.
  */
-static void recompute_msgmni(struct ipc_namespace *ns)
+void recompute_msgmni(struct ipc_namespace *ns)
 {
 	struct sysinfo i;
 	unsigned long allowed;
diff --git a/ipc/namespace.c b/ipc/namespace.c
index fe3c97aa99d..f7a35be2e77 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -26,6 +26,8 @@ static struct ipc_namespace *clone_ipc_ns(struct ipc_namespace *old_ns)
 	msg_init_ns(ns);
 	shm_init_ns(ns);
 
+	register_ipcns_notifier(ns);
+
 	kref_init(&ns->kref);
 	return ns;
 }
@@ -81,6 +83,15 @@ void free_ipc_ns(struct kref *kref)
 	struct ipc_namespace *ns;
 
 	ns = container_of(kref, struct ipc_namespace, kref);
+	/*
+	 * Unregistering the hotplug notifier at the beginning guarantees
+	 * that the ipc namespace won't be freed while we are inside the
+	 * callback routine. Since the blocking_notifier_chain_XXX routines
+	 * hold a rw lock on the notifier list, unregister_ipcns_notifier()
+	 * won't take the rw lock before blocking_notifier_call_chain() has
+	 * released the rd lock.
+	 */
+	unregister_ipcns_notifier(ns);
 	sem_exit_ns(ns);
 	msg_exit_ns(ns);
 	shm_exit_ns(ns);
diff --git a/ipc/util.c b/ipc/util.c
index c27f0e92f48..2d545d7144a 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -33,6 +33,7 @@
 #include <linux/audit.h>
 #include <linux/nsproxy.h>
 #include <linux/rwsem.h>
+#include <linux/memory.h>
 #include <linux/ipc_namespace.h>
 
 #include <asm/unistd.h>
@@ -55,11 +56,41 @@ struct ipc_namespace init_ipc_ns = {
 atomic_t nr_ipc_ns = ATOMIC_INIT(1);
 
 
+#ifdef CONFIG_MEMORY_HOTPLUG
+
+static int ipc_memory_callback(struct notifier_block *self,
+				unsigned long action, void *arg)
+{
+	switch (action) {
+	case MEM_ONLINE:    /* memory successfully brought online */
+	case MEM_OFFLINE:   /* or offline: it's time to recompute msgmni */
+		/*
+		 * This is done by invoking the ipcns notifier chain with the
+		 * IPC_MEMCHANGED event.
+		 */
+		ipcns_notify(IPCNS_MEMCHANGED);
+		break;
+	case MEM_GOING_ONLINE:
+	case MEM_GOING_OFFLINE:
+	case MEM_CANCEL_ONLINE:
+	case MEM_CANCEL_OFFLINE:
+	default:
+		break;
+	}
+
+	return NOTIFY_OK;
+}
+
+#endif /* CONFIG_MEMORY_HOTPLUG */
+
 /**
  *	ipc_init	-	initialise IPC subsystem
  *
  *	The various system5 IPC resources (semaphores, messages and shared
  *	memory) are initialised
+ *	A callback routine is registered into the memory hotplug notifier
+ *	chain: since msgmni scales to lowmem this callback routine will be
+ *	called upon successful memory add / remove to recompute msmgni.
  */
  
 static int __init ipc_init(void)
@@ -67,6 +98,8 @@ static int __init ipc_init(void)
 	sem_init();
 	msg_init();
 	shm_init();
+	hotplug_memory_notifier(ipc_memory_callback, IPC_CALLBACK_PRI);
+	register_ipcns_notifier(&init_ipc_ns);
 	return 0;
 }
 __initcall(ipc_init);
diff --git a/ipc/util.h b/ipc/util.h
index f37d160c98f..0e3d79037a2 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -124,6 +124,8 @@ extern void free_msg(struct msg_msg *msg);
 extern struct msg_msg *load_msg(const void __user *src, int len);
 extern int store_msg(void __user *dest, struct msg_msg *msg, int len);
 
+extern void recompute_msgmni(struct ipc_namespace *);
+
 static inline int ipc_buildid(int id, int seq)
 {
 	return SEQ_MULTIPLIER * seq + id;