Re: [PATCH 1/1] IPC - Do not use a negative value to re-enable msgmni automatic recomputing

Nadia Derbey
2008-07-04T05:28:33Z

Matt Helsley wrote:
> On Thu, 2008-07-03 at 14:15 +0200, Nadia.Derbey@bull.net wrote:
>
>>plain text document attachment (auto_msgmni_proc_file.patch)
>>[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-rc5-mm3.
>
>
> This makes sense to me.
>
>
>>Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>
>>
>>---
>> include/linux/ipc_namespace.h | 1
>> ipc/ipc_sysctl.c | 75 ++++++++++++++++++++++++++++++++++--------
>> ipc/ipcns_notifier.c | 19 ++++++++--
>> 3 files changed, 78 insertions(+), 17 deletions(-)
>>
>>Index: linux-2.6.26-rc5-mm3/ipc/ipc_sysctl.c
>>===================================================================
>>--- linux-2.6.26-rc5-mm3.orig/ipc/ipc_sysctl.c 2008-06-16 09:12:57.000000000 +0200
>>+++ linux-2.6.26-rc5-mm3/ipc/ipc_sysctl.c 2008-07-03 13:29:50.000000000 +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 +73,15 @@ 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 and it has a
>>+ * callback routine registered on the ipc namespace notifier
>>+ * chain: we don't want this tunable to be recomputed anymore
>>+ * upon memory add/remove or ipc namespace creation/removal.
>>+ * It can come back to a recomputable state if the
>>+ * corresponding auto_ file is set to 1.
>>+ */
>
>
> The register_ipcns_notifier() code tells us what will trigger the
> recalculation. If that code gets changed you'd need to update this
> comment. Also your comment at the top of the function describes what 0/1
> mean when written to this file. So I think this comment could be greatly
> simplified:
>
> /*
> * Disabling automatic adjustment of msgmni simply requires
> * unregistering the notifiers that trigger recalculation.
> */
>
>
>>+ unregister_ipcns_notifier(current->nsproxy->ipc_ns);
>>
>> return rc;
>> }
>>@@ -87,10 +97,39 @@ 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 +181,11 @@ 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 +194,9 @@ 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 +261,16 @@ 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,
>>+ },
>> {}
>> };
>>
>>Index: linux-2.6.26-rc5-mm3/include/linux/ipc_namespace.h
>>===================================================================
>>--- linux-2.6.26-rc5-mm3.orig/include/linux/ipc_namespace.h 2008-06-16 09:12:03.000000000 +0200
>>+++ linux-2.6.26-rc5-mm3/include/linux/ipc_namespace.h 2008-07-03 08:33:56.000000000 +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;
>>Index: linux-2.6.26-rc5-mm3/ipc/ipcns_notifier.c
>>===================================================================
>>--- linux-2.6.26-rc5-mm3.orig/ipc/ipcns_notifier.c 2008-06-16 09:12:57.000000000 +0200
>>+++ linux-2.6.26-rc5-mm3/ipc/ipcns_notifier.c 2008-07-03 11:38:07.000000000 +0200
>>@@ -55,25 +55,36 @@ 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)
>> {
>>- return blocking_notifier_chain_unregister(&ipcns_chain,
>>- &ns->ipcns_nb);
>>+ blocking_notifier_chain_unregister(&ipcns_chain, &ns->ipcns_nb);
>>+ ns->auto_msgmni = 0;
>>+ return 0;
>> }
>
>
> This looks odd -- we're no longer returning the return code from
> blocking_notifier_chain_unregister().
> From what I can see in the patch,
> the return value is unused. Perhaps it ought to be removed? Otherwise it
> might make sense to do the same as you did with "register":
>
> rc = blocking_notifier_chain_unregister(&ipcns_chain, &ns->ipcns_nb);
> if (!rc)
> ns->auto_msgmni = 0;
> return rc;
>
> Cheers,
> -Matt Helsley
>
>
>

Well, I thought that the registration routines might evolve in the
future, even though they are now unconditionally returning 0. That's why
I'm setting the flag to 1 only if
blocking_notifier_chain_cond_register() succeeded.
Now, talking about unregister: it acutally returns a ENOENT, but there's
no need to test the return code, since even in that case the flag should
be set to 0.
I'll change unregister_ipcns_notifier() into a void.

Thanks for the review, Matt!

Regards,
Nadia