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Subject[PATCH 1/1] IPC - Do not use a negative value to re-enable msgmni automatic recomputing
Resending after fixing the issues pointed out by Matt.
Now applies to 2.6.26-rc8-mm1.

[PATCH 01/01]

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.
This patch applies to 2.6.26-rc8-mm1.

Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>

---
include/linux/ipc_namespace.h | 3 +
ipc/ipc_sysctl.c | 72 ++++++++++++++++++++++++++++++++++--------
ipc/ipcns_notifier.c | 20 ++++++++---
3 files changed, 76 insertions(+), 19 deletions(-)
Index: linux-2.6.26-rc8-mm1/include/linux/ipc_namespace.h
===================================================================
--- linux-2.6.26-rc8-mm1.orig/include/linux/ipc_namespace.h 2008-07-03 16:10:39.000000000 +0200
+++ linux-2.6.26-rc8-mm1/include/linux/ipc_namespace.h 2008-07-04 08:10:50.000000000 +0200
@@ -36,6 +36,7 @@ struct ipc_namespace {
int msg_ctlmni;
atomic_t msg_bytes;
atomic_t msg_hdrs;
+ int auto_msgmni;

size_t shm_ctlmax;
size_t shm_ctlall;
@@ -53,7 +54,7 @@ extern atomic_t nr_ipc_ns;

extern int register_ipcns_notifier(struct ipc_namespace *);
extern int cond_register_ipcns_notifier(struct ipc_namespace *);
-extern int unregister_ipcns_notifier(struct ipc_namespace *);
+extern void unregister_ipcns_notifier(struct ipc_namespace *);
extern int ipcns_notify(unsigned long);

#else /* CONFIG_SYSVIPC */
Index: linux-2.6.26-rc8-mm1/ipc/ipcns_notifier.c
===================================================================
--- linux-2.6.26-rc8-mm1.orig/ipc/ipcns_notifier.c 2008-07-03 16:10:39.000000000 +0200
+++ linux-2.6.26-rc8-mm1/ipc/ipcns_notifier.c 2008-07-04 08:12:53.000000000 +0200
@@ -55,25 +55,35 @@ static int ipcns_callback(struct notifie

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)
Index: linux-2.6.26-rc8-mm1/ipc/ipc_sysctl.c
===================================================================
--- linux-2.6.26-rc8-mm1.orig/ipc/ipc_sysctl.c 2008-07-03 16:10:39.000000000 +0200
+++ linux-2.6.26-rc8-mm1/ipc/ipc_sysctl.c 2008-07-04 08:16:36.000000000 +0200
@@ -27,15 +27,17 @@ static void *get_ipc(ctl_table *table)
}

/*
- * Routine that is called when a tunable has successfully been changed by
- * hand and it has a callback routine registered on the ipc namespace notifier
- * chain: we don't want such tunables to be recomputed anymore upon memory
- * add/remove or ipc namespace creation/removal.
- * They can come back to a recomputable state by being set to a <0 value.
+ * Routine that is called when the file "auto_msgmni" has successfully been
+ * written.
+ * Two values are allowed:
+ * 0: unregister msgmni's callback routine from the ipc namespace notifier
+ * chain. This means that msgmni won't be recomputed anymore upon memory
+ * add/remove or ipc namespace creation/removal.
+ * 1: register back the callback routine.
*/
-static void tunable_set_callback(int val)
+static void ipc_auto_callback(int val)
{
- if (val >= 0)
+ if (!val)
unregister_ipcns_notifier(current->nsproxy->ipc_ns);
else {
/*
@@ -71,7 +73,12 @@ static int proc_ipc_callback_dointvec(ct
rc = proc_dointvec(&ipc_table, write, filp, buffer, lenp, ppos);

if (write && !rc && lenp_bef == *lenp)
- tunable_set_callback(*((int *)(ipc_table.data)));
+ /*
+ * Tunable has successfully been changed by hand. Disable its
+ * automatic adjustment. This simply requires unregistering
+ * the notifiers that trigger recalculation.
+ */
+ unregister_ipcns_notifier(current->nsproxy->ipc_ns);

return rc;
}
@@ -87,10 +94,39 @@ static int proc_ipc_doulongvec_minmax(ct
lenp, ppos);
}

+static int proc_ipcauto_dointvec_minmax(ctl_table *table, int write,
+ struct file *filp, void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ struct ctl_table ipc_table;
+ size_t lenp_bef = *lenp;
+ int oldval;
+ int rc;
+
+ memcpy(&ipc_table, table, sizeof(ipc_table));
+ ipc_table.data = get_ipc(table);
+ oldval = *((int *)(ipc_table.data));
+
+ rc = proc_dointvec_minmax(&ipc_table, write, filp, buffer, lenp, ppos);
+
+ if (write && !rc && lenp_bef == *lenp) {
+ int newval = *((int *)(ipc_table.data));
+ /*
+ * The file "auto_msgmni" has correctly been set.
+ * React by (un)registering the corresponding tunable, if the
+ * value has changed.
+ */
+ if (newval != oldval)
+ ipc_auto_callback(newval);
+ }
+
+ return rc;
+}
+
#else
#define proc_ipc_doulongvec_minmax NULL
#define proc_ipc_dointvec NULL
#define proc_ipc_callback_dointvec NULL
+#define proc_ipcauto_dointvec_minmax NULL
#endif

#ifdef CONFIG_SYSCTL_SYSCALL
@@ -142,14 +178,11 @@ static int sysctl_ipc_registered_data(ct
rc = sysctl_ipc_data(table, name, nlen, oldval, oldlenp, newval,
newlen);

- if (newval && newlen && rc > 0) {
+ if (newval && newlen && rc > 0)
/*
* Tunable has successfully been changed from userland
*/
- int *data = get_ipc(table);
-
- tunable_set_callback(*data);
- }
+ unregister_ipcns_notifier(current->nsproxy->ipc_ns);

return rc;
}
@@ -158,6 +191,9 @@ static int sysctl_ipc_registered_data(ct
#define sysctl_ipc_registered_data NULL
#endif

+static int zero;
+static int one = 1;
+
static struct ctl_table ipc_kern_table[] = {
{
.ctl_name = KERN_SHMMAX,
@@ -222,6 +258,16 @@ static struct ctl_table ipc_kern_table[]
.proc_handler = proc_ipc_dointvec,
.strategy = sysctl_ipc_data,
},
+ {
+ .ctl_name = CTL_UNNUMBERED,
+ .procname = "auto_msgmni",
+ .data = &init_ipc_ns.auto_msgmni,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_ipcauto_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
{}
};

--


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