[PATCH v3] PM: suspend: Add sysfs attribute to control the "sync on suspend" behavior

[PATCH v3] PM: suspend: Add sysfs attribute to control the "sync on suspend" behavior

[Jonas Meurer]

The sysfs attribute `/sys/power/sync_on_suspend` controls, whether or not
filesystems are synced by the kernel before system suspend.

Congruously, the behaviour of build-time switch CONFIG_SUSPEND_SKIP_SYNC
is slightly changed: It now defines the run-tim default for the new sysfs
attribute `/sys/power/sync_on_suspend`.

The run-time attribute is added because the existing corresponding
build-time Kconfig flag for (`CONFIG_SUSPEND_SKIP_SYNC`) is not flexible
enough. E.g. Linux distributions that provide pre-compiled kernels
usually want to stick with the default (sync filesystems before suspend)
but under special conditions this needs to be changed.

One example for such a special condition is user-space handling of
suspending block devices (e.g. using `cryptsetup luksSuspend` or `dmsetup
suspend`) before system suspend. The Kernel trying to sync filesystems
after the underlying block device already got suspended obviously leads
to dead-locks. Be aware that you have to take care of the filesystem sync
yourself before suspending the system in those scenarios.

Signed-off-by: Jonas Meurer <jonas@freesources.org>
---
 Documentation/ABI/testing/sysfs-power | 13 +++++++++++
 include/linux/suspend.h               |  2 ++
 kernel/power/Kconfig                  |  5 +++-
 kernel/power/main.c                   | 33 +++++++++++++++++++++++++++
 kernel/power/suspend.c                |  2 +-
 5 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-power b/Documentation/ABI/testing/sysfs-power
index 6f87b9dd384b..9392a83afce8 100644
--- a/Documentation/ABI/testing/sysfs-power
+++ b/Documentation/ABI/testing/sysfs-power
@@ -407,3 +407,16 @@ Contact:	Kalesh Singh <kaleshsingh96@gmail.com>
 Description:
 		The /sys/power/suspend_stats/last_failed_step file contains
 		the last failed step in the suspend/resume path.
+
+What:		/sys/power/sync_on_suspend
+Date:		October 2019
+Contact:	Jonas Meurer <jonas@freesources.org>
+Description:
+		This file controls whether or not the kernel will sync()
+		filesystems during system suspend (after freezing user space
+		and before suspending devices).
+
+		Writing a "1" to this file enables the sync() and writing a
+		"0" disables it.  Reads from the file return the current value.
+		The default is "1" but can be configured with the build-time
+		config flag "SUSPEND_SKIP_SYNC".
diff --git a/include/linux/suspend.h b/include/linux/suspend.h
index 6fc8843f1c9e..4a230c2f1c31 100644
--- a/include/linux/suspend.h
+++ b/include/linux/suspend.h
@@ -329,6 +329,7 @@ extern void arch_suspend_disable_irqs(void);
 extern void arch_suspend_enable_irqs(void);
 
 extern int pm_suspend(suspend_state_t state);
+extern bool sync_on_suspend_enabled;
 #else /* !CONFIG_SUSPEND */
 #define suspend_valid_only_mem	NULL
 
@@ -342,6 +343,7 @@ static inline bool pm_suspend_default_s2idle(void) { return false; }
 
 static inline void suspend_set_ops(const struct platform_suspend_ops *ops) {}
 static inline int pm_suspend(suspend_state_t state) { return -ENOSYS; }
+static inline bool sync_on_suspend_enabled(void) { return true; }
 static inline bool idle_should_enter_s2idle(void) { return false; }
 static inline void __init pm_states_init(void) {}
 static inline void s2idle_set_ops(const struct platform_s2idle_ops *ops) {}
diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
index d3667b4075c1..7cbfbeacd68a 100644
--- a/kernel/power/Kconfig
+++ b/kernel/power/Kconfig
@@ -27,7 +27,10 @@ config SUSPEND_SKIP_SYNC
 	  Skip the kernel sys_sync() before freezing user processes.
 	  Some systems prefer not to pay this cost on every invocation
 	  of suspend, or they are content with invoking sync() from
-	  user-space before invoking suspend.  Say Y if that's your case.
+	  user-space before invoking suspend.  There's a run-time switch
+	  at '/sys/power/sync_on_suspend' to configure this behaviour.
+	  This setting changes the default for the run-tim switch. Say Y
+	  to change the default to disable the kernel sys_sync().
 
 config HIBERNATE_CALLBACKS
 	bool
diff --git a/kernel/power/main.c b/kernel/power/main.c
index e26de7af520b..69b7a8aeca3b 100644
--- a/kernel/power/main.c
+++ b/kernel/power/main.c
@@ -190,6 +190,38 @@ static ssize_t mem_sleep_store(struct kobject *kobj, struct kobj_attribute *attr
 }
 
 power_attr(mem_sleep);
+
+/*
+ * sync_on_suspend: invoke ksys_sync_helper() before suspend.
+ *
+ * show() returns whether ksys_sync_helper() is invoked before suspend.
+ * store() accepts 0 or 1.  0 disables ksys_sync_helper() and 1 enables it.
+ */
+bool sync_on_suspend_enabled = !IS_ENABLED(CONFIG_SUSPEND_SKIP_SYNC);
+
+static ssize_t sync_on_suspend_show(struct kobject *kobj,
+				   struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", sync_on_suspend_enabled);
+}
+
+static ssize_t sync_on_suspend_store(struct kobject *kobj,
+				    struct kobj_attribute *attr,
+				    const char *buf, size_t n)
+{
+	unsigned long val;
+
+	if (kstrtoul(buf, 10, &val))
+		return -EINVAL;
+
+	if (val > 1)
+		return -EINVAL;
+
+	sync_on_suspend_enabled = !!val;
+	return n;
+}
+
+power_attr(sync_on_suspend);
 #endif /* CONFIG_SUSPEND */
 
 #ifdef CONFIG_PM_SLEEP_DEBUG
@@ -855,6 +887,7 @@ static struct attribute * g[] = {
 	&wakeup_count_attr.attr,
 #ifdef CONFIG_SUSPEND
 	&mem_sleep_attr.attr,
+	&sync_on_suspend_attr.attr,
 #endif
 #ifdef CONFIG_PM_AUTOSLEEP
 	&autosleep_attr.attr,
diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
index f3b7239f1892..2c47280fbfc7 100644
--- a/kernel/power/suspend.c
+++ b/kernel/power/suspend.c
@@ -564,7 +564,7 @@ static int enter_state(suspend_state_t state)
 	if (state == PM_SUSPEND_TO_IDLE)
 		s2idle_begin();
 
-	if (!IS_ENABLED(CONFIG_SUSPEND_SKIP_SYNC)) {
+	if (sync_on_suspend_enabled) {
 		trace_suspend_resume(TPS("sync_filesystems"), 0, true);
 		ksys_sync_helper();
 		trace_suspend_resume(TPS("sync_filesystems"), 0, false);
-- 
2.20.1

Re: [PATCH v3] PM: suspend: Add sysfs attribute to control the "sync on suspend" behavior

[Rafael J. Wysocki]

On Thu, Jan 16, 2020 at 12:53 PM Jonas Meurer <jonas@freesources.org> wrote:
>
> The sysfs attribute `/sys/power/sync_on_suspend` controls, whether or not
> filesystems are synced by the kernel before system suspend.
>
> Congruously, the behaviour of build-time switch CONFIG_SUSPEND_SKIP_SYNC
> is slightly changed: It now defines the run-tim default for the new sysfs
> attribute `/sys/power/sync_on_suspend`.
>
> The run-time attribute is added because the existing corresponding
> build-time Kconfig flag for (`CONFIG_SUSPEND_SKIP_SYNC`) is not flexible
> enough. E.g. Linux distributions that provide pre-compiled kernels
> usually want to stick with the default (sync filesystems before suspend)
> but under special conditions this needs to be changed.
>
> One example for such a special condition is user-space handling of
> suspending block devices (e.g. using `cryptsetup luksSuspend` or `dmsetup
> suspend`) before system suspend. The Kernel trying to sync filesystems
> after the underlying block device already got suspended obviously leads
> to dead-locks. Be aware that you have to take care of the filesystem sync
> yourself before suspending the system in those scenarios.
>
> Signed-off-by: Jonas Meurer <jonas@freesources.org>

Applied as 5.6 material with minor changes in the ABI document, thanks!

> ---
>  Documentation/ABI/testing/sysfs-power | 13 +++++++++++
>  include/linux/suspend.h               |  2 ++
>  kernel/power/Kconfig                  |  5 +++-
>  kernel/power/main.c                   | 33 +++++++++++++++++++++++++++
>  kernel/power/suspend.c                |  2 +-
>  5 files changed, 53 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-power b/Documentation/ABI/testing/sysfs-power
> index 6f87b9dd384b..9392a83afce8 100644
> --- a/Documentation/ABI/testing/sysfs-power
> +++ b/Documentation/ABI/testing/sysfs-power
> @@ -407,3 +407,16 @@ Contact:   Kalesh Singh <kaleshsingh96@gmail.com>
>  Description:
>                 The /sys/power/suspend_stats/last_failed_step file contains
>                 the last failed step in the suspend/resume path.
> +
> +What:          /sys/power/sync_on_suspend
> +Date:          October 2019
> +Contact:       Jonas Meurer <jonas@freesources.org>
> +Description:
> +               This file controls whether or not the kernel will sync()
> +               filesystems during system suspend (after freezing user space
> +               and before suspending devices).
> +
> +               Writing a "1" to this file enables the sync() and writing a
> +               "0" disables it.  Reads from the file return the current value.
> +               The default is "1" but can be configured with the build-time
> +               config flag "SUSPEND_SKIP_SYNC".
> diff --git a/include/linux/suspend.h b/include/linux/suspend.h
> index 6fc8843f1c9e..4a230c2f1c31 100644
> --- a/include/linux/suspend.h
> +++ b/include/linux/suspend.h
> @@ -329,6 +329,7 @@ extern void arch_suspend_disable_irqs(void);
>  extern void arch_suspend_enable_irqs(void);
>
>  extern int pm_suspend(suspend_state_t state);
> +extern bool sync_on_suspend_enabled;
>  #else /* !CONFIG_SUSPEND */
>  #define suspend_valid_only_mem NULL
>
> @@ -342,6 +343,7 @@ static inline bool pm_suspend_default_s2idle(void) { return false; }
>
>  static inline void suspend_set_ops(const struct platform_suspend_ops *ops) {}
>  static inline int pm_suspend(suspend_state_t state) { return -ENOSYS; }
> +static inline bool sync_on_suspend_enabled(void) { return true; }
>  static inline bool idle_should_enter_s2idle(void) { return false; }
>  static inline void __init pm_states_init(void) {}
>  static inline void s2idle_set_ops(const struct platform_s2idle_ops *ops) {}
> diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
> index d3667b4075c1..7cbfbeacd68a 100644
> --- a/kernel/power/Kconfig
> +++ b/kernel/power/Kconfig
> @@ -27,7 +27,10 @@ config SUSPEND_SKIP_SYNC
>           Skip the kernel sys_sync() before freezing user processes.
>           Some systems prefer not to pay this cost on every invocation
>           of suspend, or they are content with invoking sync() from
> -         user-space before invoking suspend.  Say Y if that's your case.
> +         user-space before invoking suspend.  There's a run-time switch
> +         at '/sys/power/sync_on_suspend' to configure this behaviour.
> +         This setting changes the default for the run-tim switch. Say Y
> +         to change the default to disable the kernel sys_sync().
>
>  config HIBERNATE_CALLBACKS
>         bool
> diff --git a/kernel/power/main.c b/kernel/power/main.c
> index e26de7af520b..69b7a8aeca3b 100644
> --- a/kernel/power/main.c
> +++ b/kernel/power/main.c
> @@ -190,6 +190,38 @@ static ssize_t mem_sleep_store(struct kobject *kobj, struct kobj_attribute *attr
>  }
>
>  power_attr(mem_sleep);
> +
> +/*
> + * sync_on_suspend: invoke ksys_sync_helper() before suspend.
> + *
> + * show() returns whether ksys_sync_helper() is invoked before suspend.
> + * store() accepts 0 or 1.  0 disables ksys_sync_helper() and 1 enables it.
> + */
> +bool sync_on_suspend_enabled = !IS_ENABLED(CONFIG_SUSPEND_SKIP_SYNC);
> +
> +static ssize_t sync_on_suspend_show(struct kobject *kobj,
> +                                  struct kobj_attribute *attr, char *buf)
> +{
> +       return sprintf(buf, "%d\n", sync_on_suspend_enabled);
> +}
> +
> +static ssize_t sync_on_suspend_store(struct kobject *kobj,
> +                                   struct kobj_attribute *attr,
> +                                   const char *buf, size_t n)
> +{
> +       unsigned long val;
> +
> +       if (kstrtoul(buf, 10, &val))
> +               return -EINVAL;
> +
> +       if (val > 1)
> +               return -EINVAL;
> +
> +       sync_on_suspend_enabled = !!val;
> +       return n;
> +}
> +
> +power_attr(sync_on_suspend);
>  #endif /* CONFIG_SUSPEND */
>
>  #ifdef CONFIG_PM_SLEEP_DEBUG
> @@ -855,6 +887,7 @@ static struct attribute * g[] = {
>         &wakeup_count_attr.attr,
>  #ifdef CONFIG_SUSPEND
>         &mem_sleep_attr.attr,
> +       &sync_on_suspend_attr.attr,
>  #endif
>  #ifdef CONFIG_PM_AUTOSLEEP
>         &autosleep_attr.attr,
> diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
> index f3b7239f1892..2c47280fbfc7 100644
> --- a/kernel/power/suspend.c
> +++ b/kernel/power/suspend.c
> @@ -564,7 +564,7 @@ static int enter_state(suspend_state_t state)
>         if (state == PM_SUSPEND_TO_IDLE)
>                 s2idle_begin();
>
> -       if (!IS_ENABLED(CONFIG_SUSPEND_SKIP_SYNC)) {
> +       if (sync_on_suspend_enabled) {
>                 trace_suspend_resume(TPS("sync_filesystems"), 0, true);
>                 ksys_sync_helper();
>                 trace_suspend_resume(TPS("sync_filesystems"), 0, false);
> --
> 2.20.1
>

Re: [PATCH v3] PM: suspend: Add sysfs attribute to control the "sync on suspend" behavior

[Jonas Meurer]

Hey Rafael,

Rafael J. Wysocki:
> On Thu, Jan 16, 2020 at 12:53 PM Jonas Meurer <jonas@freesources.org> wrote:
>>
>> The sysfs attribute `/sys/power/sync_on_suspend` controls, whether or not
>> filesystems are synced by the kernel before system suspend.
>>
>> Congruously, the behaviour of build-time switch CONFIG_SUSPEND_SKIP_SYNC
>> is slightly changed: It now defines the run-tim default for the new sysfs
>> attribute `/sys/power/sync_on_suspend`.
>>
>> The run-time attribute is added because the existing corresponding
>> build-time Kconfig flag for (`CONFIG_SUSPEND_SKIP_SYNC`) is not flexible
>> enough. E.g. Linux distributions that provide pre-compiled kernels
>> usually want to stick with the default (sync filesystems before suspend)
>> but under special conditions this needs to be changed.
>>
>> One example for such a special condition is user-space handling of
>> suspending block devices (e.g. using `cryptsetup luksSuspend` or `dmsetup
>> suspend`) before system suspend. The Kernel trying to sync filesystems
>> after the underlying block device already got suspended obviously leads
>> to dead-locks. Be aware that you have to take care of the filesystem sync
>> yourself before suspending the system in those scenarios.
>>
>> Signed-off-by: Jonas Meurer <jonas@freesources.org>
> 
> Applied as 5.6 material with minor changes in the ABI document, thanks!

Wow, that's great news. I'm excited to read that the patch finally will
enter the Linux kernel \o/

Thanks a lot for guiding me through my first Linux kernel patch
contribution.

Cheers
 jonas

Re: [PATCH v3] PM: suspend: Add sysfs attribute to control the "sync on suspend" behavior

[Pavel Machek]

Hi!

> > The sysfs attribute `/sys/power/sync_on_suspend` controls, whether or not
> > filesystems are synced by the kernel before system suspend.
> >
> > Congruously, the behaviour of build-time switch CONFIG_SUSPEND_SKIP_SYNC
> > is slightly changed: It now defines the run-tim default for the new sysfs
> > attribute `/sys/power/sync_on_suspend`.
> >
> > The run-time attribute is added because the existing corresponding
> > build-time Kconfig flag for (`CONFIG_SUSPEND_SKIP_SYNC`) is not flexible
> > enough. E.g. Linux distributions that provide pre-compiled kernels
> > usually want to stick with the default (sync filesystems before suspend)
> > but under special conditions this needs to be changed.
> >
> > One example for such a special condition is user-space handling of
> > suspending block devices (e.g. using `cryptsetup luksSuspend` or `dmsetup
> > suspend`) before system suspend. The Kernel trying to sync filesystems
> > after the underlying block device already got suspended obviously leads
> > to dead-locks. Be aware that you have to take care of the filesystem sync
> > yourself before suspending the system in those scenarios.
> >
> > Signed-off-by: Jonas Meurer <jonas@freesources.org>
> 
> Applied as 5.6 material with minor changes in the ABI document, thanks!

For the record, I still believe this is bad idea.

User should not have to tweak variables in /sys for system not to
deadlock with cryptsetup.. and we are stuck with this pretty much
forever.

									Pavel