[PATCH v4 3/3] PM: hibernate: fix spurious GFP mask WARNING in uswsusp path

[Youngjun Park <youngjun.park@lge.com>]

Commit 35e4a69b2003f ("PM: sleep: Allow pm_restrict_gfp_mask()
stacking") introduced refcount-based GFP mask management that warns
when pm_restore_gfp_mask() is called with saved_gfp_count == 0:

  WARNING: kernel/power/main.c:44 at pm_restore_gfp_mask+0xd7/0xf0
  CPU: 0 UID: 0 PID: 373 Comm: s2disk
  Call Trace:
   snapshot_ioctl+0x964/0xbd0
   __x64_sys_ioctl+0x724/0x1320
  ...

The uswsusp path calls pm_restore_gfp_mask() defensively in
SNAPSHOT_CREATE_IMAGE and SNAPSHOT_UNFREEZE where the GFP mask may
or may not be restricted depending on context (first call vs retry,
hibernate vs resume). Before the stacking patch this was a silent
no-op; now it triggers a WARNING.

Introduce pm_restore_gfp_mask_safe() that skips the call when
saved_gfp_count is 0. This is preferred over tracking the restrict
state in snapshot_ioctl, as incorrect tracking risks leaving the
GFP mask permanently restricted.

Fixes: 35e4a69b2003f ("PM: sleep: Allow pm_restrict_gfp_mask() stacking")
Signed-off-by: Youngjun Park <youngjun.park@lge.com>
---
 include/linux/suspend.h | 1 +
 kernel/power/main.c     | 7 +++++++
 kernel/power/user.c     | 4 ++--
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/suspend.h b/include/linux/suspend.h
index b02876f1ae38..7777931d88a5 100644
--- a/include/linux/suspend.h
+++ b/include/linux/suspend.h
@@ -454,6 +454,7 @@ extern void pm_report_hw_sleep_time(u64 t);
 extern void pm_report_max_hw_sleep(u64 t);
 void pm_restrict_gfp_mask(void);
 void pm_restore_gfp_mask(void);
+void pm_restore_gfp_mask_safe(void);
 
 #define pm_notifier(fn, pri) {				\
 	static struct notifier_block fn##_nb =			\
diff --git a/kernel/power/main.c b/kernel/power/main.c
index 5f8c9e12eaec..e610a8c8b7ff 100644
--- a/kernel/power/main.c
+++ b/kernel/power/main.c
@@ -36,6 +36,13 @@
 static unsigned int saved_gfp_count;
 static gfp_t saved_gfp_mask;
 
+void pm_restore_gfp_mask_safe(void)
+{
+	if (!saved_gfp_count)
+		return;
+	pm_restore_gfp_mask();
+}
+
 void pm_restore_gfp_mask(void)
 {
 	WARN_ON(!mutex_is_locked(&system_transition_mutex));
diff --git a/kernel/power/user.c b/kernel/power/user.c
index 3e41544b99d5..41cff6a89a1c 100644
--- a/kernel/power/user.c
+++ b/kernel/power/user.c
@@ -306,7 +306,7 @@ static long snapshot_ioctl(struct file *filp, unsigned int cmd,
 	case SNAPSHOT_UNFREEZE:
 		if (!data->frozen || data->ready)
 			break;
-		pm_restore_gfp_mask();
+		pm_restore_gfp_mask_safe();
 		free_basic_memory_bitmaps();
 		data->free_bitmaps = false;
 		thaw_processes();
@@ -318,7 +318,7 @@ static long snapshot_ioctl(struct file *filp, unsigned int cmd,
 			error = -EPERM;
 			break;
 		}
-		pm_restore_gfp_mask();
+		pm_restore_gfp_mask_safe();
 		error = hibernation_snapshot(data->platform_support);
 		if (!error) {
 			error = put_user(in_suspend, (int __user *)arg);
-- 
2.34.1