[PATCH] uswsusp: automatically free the in-memory image once s2disk has finished with it

[Alan Jenkins <alan-jenkins-cCz0Lq7MMjm9FHfhHBbuYA@public.gmane.org>]

The original in-kernel suspend (swsusp) frees the in-memory hibernation
image before powering off the machine.  s2disk doesn't, so there is
_much_ less free memory when it tries to power off.

This is a gratuitous difference.  The userspace suspend interface
/dev/snapshot only allows the hibernation image to be read once.
Once the s2disk program has read the last page, we can free the entire
image.

This avoids a hang after writing the hibernation image which was
triggered by commit 5f8dcc21211a3d4e3a7a5ca366b469fb88117f61
"page-allocator: split per-cpu list into one-list-per-migrate-type":

    [top of trace lost due to screen height]
    ? shrink_zone
    ? try_to_free_pages
    ? isolate_pages_global
    ? __alloc_pages_nodemask
    ? kthread
    ? __get_free_pages
    ? copy_process
    ? kthread
    ? do_fork
    ...

    INFO: task s2disk:2036 blocked for more than 120 seconds
    ...
    Call Trace:
    ...
    ? wait_for_common
    ? default_wake_function
    ? kthread_create
    ? worker_thread
    ? create_workqueue_thread
    ? worker_thread
    ? __create_workqueue_key
    ? stop_machine_create
    ? disable_nonboot_cpus
    ? hibernation_platform_enter
    ? snapshot_ioctl
    ...
    ? sys_ioctl
    ...

Signed-off-by: Alan Jenkins <alan-jenkins-cCz0Lq7MMjm9FHfhHBbuYA@public.gmane.org>
---
 kernel/power/user.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/kernel/power/user.c b/kernel/power/user.c
index bf0014d..94d0210 100644
--- a/kernel/power/user.c
+++ b/kernel/power/user.c
@@ -165,6 +165,10 @@ static ssize_t snapshot_read(struct file *filp, char __user *buf,
 			res = -EFAULT;
 		else
 			*offp = data->handle.offset;
+	} else {
+		swsusp_free();
+		memset(&data->handle, 0, sizeof(struct snapshot_handle));
+		data->ready = 0;
 	}
 
  Unlock:
-- 
1.6.3.3