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

Matt Helsley
2008-07-03T21:20:35Z

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