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ipc: do not use a negative value to re-enable msgmni automatic recomp…
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…uting

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 <[email protected]>
Cc: Solofo Ramangalahy <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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@torvalds
Nadia Derbey authored and torvalds committed Jul 25, 2008
1 parent f1a43f9 commit 9eefe52
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Showing 3 changed files with 76 additions and 19 deletions.
3 changes: 2 additions & 1 deletion include/linux/ipc_namespace.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -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 */
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72 changes: 59 additions & 13 deletions ipc/ipc_sysctl.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
/*
Expand Down Expand Up @@ -71,7 +73,12 @@ static int proc_ipc_callback_dointvec(ctl_table *table, int write,
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;
}
Expand All @@ -87,10 +94,39 @@ static int proc_ipc_doulongvec_minmax(ctl_table *table, int write,
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
Expand Down Expand Up @@ -142,14 +178,11 @@ static int sysctl_ipc_registered_data(ctl_table *table, int __user *name,
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;
}
Expand All @@ -158,6 +191,9 @@ static int sysctl_ipc_registered_data(ctl_table *table, int __user *name,
#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,
Expand Down Expand Up @@ -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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20 changes: 15 additions & 5 deletions ipc/ipcns_notifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand Down

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