ipc: do not use a negative value to re-enable msgmni automatic recomputing

This patch proposes an alternative to the "magical
positive-versus-negative number trick" Andrew complained about last week
in http://lkml.org/lkml/2008/6/24/418.

This had been introduced with the patches that scale msgmni to the amount
of lowmem.  With these patches, msgmni has a registered notification
routine that recomputes msgmni value upon memory add/remove or ipc
namespace creation/ removal.

When msgmni is changed from user space (i.e.  value written to the proc
file), that notification routine is unregistered, and the way to make it
registered back is to write a negative value into the proc file.  This is
the "magical positive-versus-negative number trick".

To fix this, a new proc file is introduced: /proc/sys/kernel/auto_msgmni.
This file acts as ON/OFF for msgmni automatic recomputing.

With this patch, the process is the following:
1) kernel boots in "automatic recomputing mode"
   /proc/sys/kernel/msgmni contains the value that has been computed (depends
                           on lowmem)
   /proc/sys/kernel/automatic_msgmni contains "1"

2) echo <val> > /proc/sys/kernel/msgmni
   . sets msg_ctlmni to <val>
   . de-activates automatic recomputing (i.e. if, say, some memory is added
     msgmni won't be recomputed anymore)
   . /proc/sys/kernel/automatic_msgmni now contains "0"

3) echo "0" > /proc/sys/kernel/automatic_msgmni
   . de-activates msgmni automatic recomputing
     this has the same effect as 2) except that msg_ctlmni's value stays
     blocked at its current value)

3) echo "1" > /proc/sys/kernel/automatic_msgmni
   . recomputes msgmni's value based on the current available memory size
     and number of ipc namespaces
   . re-activates automatic recomputing for msgmni.

Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Solofo Ramangalahy <Solofo.Ramangalahy@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 5 deletions

diff --git a/ipc/ipcns_notifier.c b/ipc/ipcns_notifier.c
index 70ff09183f7b..b9b31a4f77e1 100644
--- a/ipc/ipcns_notifier.c
+++ b/ipc/ipcns_notifier.c
@@ -55,25 +55,35 @@ static int ipcns_callback(struct notifier_block *self,
 
 int register_ipcns_notifier(struct ipc_namespace *ns)
 {
+        int rc;
+
         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);
+        rc = blocking_notifier_chain_register(&ipcns_chain, &ns->ipcns_nb);
+        if (!rc)
+                ns->auto_msgmni = 1;
+        return rc;
 }
 
 int cond_register_ipcns_notifier(struct ipc_namespace *ns)
 {
+        int rc;
+
         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_cond_register(&ipcns_chain,
+        rc = blocking_notifier_chain_cond_register(&ipcns_chain,
                                                         &ns->ipcns_nb);
+        if (!rc)
+                ns->auto_msgmni = 1;
+        return rc;
 }
 
-int unregister_ipcns_notifier(struct ipc_namespace *ns)
+void unregister_ipcns_notifier(struct ipc_namespace *ns)
 {
-        return blocking_notifier_chain_unregister(&ipcns_chain,
-                                                &ns->ipcns_nb);
+        blocking_notifier_chain_unregister(&ipcns_chain, &ns->ipcns_nb);
+        ns->auto_msgmni = 0;
 }
 
 int ipcns_notify(unsigned long val)