PM: hibernate: shrink shmem pages after dev_pm_ops.prepare()

When hibernate with data center dGPUs, huge number of VRAM data will be
moved to shmem during dev_pm_ops.prepare(). These shmem pages take a lot
of system memory so that there's no enough free memory for creating the
hibernation image. This will cause hibernation fail and abort.

After dev_pm_ops.prepare(), call shrink_all_memory() to force move shmem
pages to swap disk and reclaim the pages, so that there's enough system
memory for hibernation image and less pages needed to copy to the image.

This patch can only flush and free about half shmem pages. It will be
better to flush and free more pages, even all of shmem pages, so that
there're less pages to be copied to the hibernation image and the overall
hibernation time can be reduced.

Signed-off-by: Samuel Zhang <guoqing.zhang@amd.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20250710062313.3226149-4-guoqing.zhang@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>

Author: guoqzhan

kernel/power/hibernate.c

@@ -381,6 +381,23 @@ static int create_image(int platform_mode)
 	return error;
 }
 
+static void shrink_shmem_memory(void)
+{
+	struct sysinfo info;
+	unsigned long nr_shmem_pages, nr_freed_pages;
+
+	si_meminfo(&info);
+	nr_shmem_pages = info.sharedram; /* current page count used for shmem */
+	/*
+	 * The intent is to reclaim all shmem pages. Though shrink_all_memory() can
+	 * only reclaim about half of them, it's enough for creating the hibernation
+	 * image.
+	 */
+	nr_freed_pages = shrink_all_memory(nr_shmem_pages);
+	pr_debug("requested to reclaim %lu shmem pages, actually freed %lu pages\n",
+			nr_shmem_pages, nr_freed_pages);
+}
+
 /**
  * hibernation_snapshot - Quiesce devices and create a hibernation image.
  * @platform_mode: If set, use platform driver to prepare for the transition.
@@ -422,6 +439,15 @@ int hibernation_snapshot(int platform_mode)
 		goto Thaw;
 	}
 
+	/*
+	 * Device drivers may move lots of data to shmem in dpm_prepare(). The shmem
+	 * pages will use lots of system memory, causing hibernation image creation
+	 * fail due to insufficient free memory.
+	 * This call is to force flush the shmem pages to swap disk and reclaim
+	 * the system memory so that image creation can succeed.
+	 */
+	shrink_shmem_memory();
+
 	console_suspend_all();
 	pm_restrict_gfp_mask();