Date:	Thu, 24 Jun 2010 18:27:34 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	pavel@....cz, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, jirislaby@...il.com
Subject: Re: [PATCH 3/9] PM / Hibernate: user, implement user_ops writer

On Wednesday, June 02, 2010, Jiri Slaby wrote:
> Switch /dev/snapshot writer to hibernate_io_ops approach so that we
> can do whatever we want with snapshot processing code. All the later
> code changes will be transparent and needn't care about different
> readers/writers.

Well.  I don't want image data to undergo _any_ transformations within the
kernel before going to s2disk.  In fact, s2disk is supposed to do whatever it
wants with the image data (that are supposed to be _raw_ image data).

So, I don't think we need to "switch" /dev/snapshot to anything adding
complexity in the process.

> In this patch only writer is implemented -- for better bisectability
> if something breaks. (The development was easier too, because one
> could break only one part, not both.)
> 
> Also, when using this interface, switch to the ops on open/release,
> so they are used.
> 
> There are explicit barriers protecting to_do_buffer, because it is
> all done in a producer-consumer fashion:
> PRODUCER (snapshot layer)
> 1) to_do_buffer is set
> 2) set TODO bit
> 3) wake CONSUMER
> 4) wait for TODO bit _clear_
> 
> CONSUMER (fops->read)
> 1) wait for TODO bit
> 2) pass to_do_buffer to userspace
> 3) clear TODO bit
> 4) wake PRODUCER
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: "Rafael J. Wysocki" <rjw@...k.pl>
> ---
>  kernel/power/user.c |  140 ++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 files changed, 132 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/power/user.c b/kernel/power/user.c
> index e819e17..b4610c3 100644
> --- a/kernel/power/user.c
> +++ b/kernel/power/user.c
> @@ -23,6 +23,8 @@
>  #include <linux/console.h>
>  #include <linux/cpu.h>
>  #include <linux/freezer.h>
> +#include <linux/mutex.h>
> +#include <linux/wait.h>
>  #include <scsi/scsi_scan.h>
>  
>  #include <asm/uaccess.h>
> @@ -61,10 +63,93 @@ static struct snapshot_data {
>  	char frozen;
>  	char ready;
>  	char platform_support;
> +	struct hibernate_io_ops *prev_ops;
>  } snapshot_state;
>  
>  atomic_t snapshot_device_available = ATOMIC_INIT(1);
>  
> +static struct {
> +	void *buffer; /* buffer to pass through */
> +	struct workqueue_struct *worker; /* the thread initiating read/write */
> +	wait_queue_head_t wait; /* wait for buffer */
> +	wait_queue_head_t done; /* read/write done? */
> +	struct mutex lock; /* protection for buffer */
> +
> +#define TODO_WORK	1
> +#define TODO_FINISH	2
> +#define TODO_CLOSED	3
> +#define TODO_ERROR	4
> +	DECLARE_BITMAP(flags, 10); /* TODO_* flags defined above */
> +} to_do;
> +
> +static unsigned long user_free_space(void)
> +{
> +	return ~0UL; /* we have no idea, maybe we will fail later */
> +}
> +
> +static struct hibernate_io_handle *user_writer_start(void)
> +{
> +	return hib_io_handle_alloc(0) ? : ERR_PTR(-ENOMEM);
> +}
> +
> +static int user_write_page(struct hibernate_io_handle *io_handle, void *buf,
> +		struct bio **bio_chain)
> +{
> +	int err = 0;
> +
> +	if (test_bit(TODO_CLOSED, to_do.flags))
> +		return -EIO;
> +
> +	mutex_lock(&to_do.lock);
> +	to_do.buffer = buf;
> +	mutex_unlock(&to_do.lock);
> +	set_bit(TODO_WORK, to_do.flags);
> +	wake_up_interruptible(&to_do.wait);
> +
> +	wait_event(to_do.done, !test_bit(TODO_WORK, to_do.flags) ||
> +			(err = test_bit(TODO_CLOSED, to_do.flags)));
> +
> +	return err ? -EIO : 0;
> +}
> +
> +static int user_writer_finish(struct hibernate_io_handle *io_handle,
> +		unsigned int flags, int error)
> +{
> +	int err = 0;
> +
> +	if (error)
> +		set_bit(TODO_ERROR, to_do.flags);
> +	set_bit(TODO_FINISH, to_do.flags);
> +	wake_up_interruptible(&to_do.wait);
> +
> +	wait_event(to_do.done, !test_bit(TODO_FINISH, to_do.flags) ||
> +			(err = test_bit(TODO_CLOSED, to_do.flags)));
> +
> +	if (!error && err)
> +		error = -EIO;
> +
> +	return error;
> +}
> +
> +struct hibernate_io_ops user_ops = {
> +	.free_space = user_free_space,
> +
> +	.writer_start = user_writer_start,
> +	.writer_finish = user_writer_finish,
> +	.write_page = user_write_page,
> +};
> +
> +static void snapshot_writer(struct work_struct *work)
> +{
> +	int ret;
> +
> +	ret = swsusp_write(0);
> +	if (ret)
> +		printk(KERN_ERR "PM: write failed with %d\n", ret);
> +}
> +
> +static DECLARE_WORK(snapshot_writer_w, snapshot_writer);
> +
>  static int snapshot_open(struct inode *inode, struct file *filp)
>  {
>  	struct snapshot_data *data;
> @@ -115,9 +200,17 @@ static int snapshot_open(struct inode *inode, struct file *filp)
>  	}
>  	if (error)
>  		atomic_inc(&snapshot_device_available);
> +	else {
> +		data->prev_ops = hibernate_io_ops;
> +		hibernate_io_ops = &user_ops;
> +	}
>  	data->frozen = 0;
>  	data->ready = 0;
>  	data->platform_support = 0;
> +	memset(to_do.flags, 0, sizeof(*to_do.flags));
> +	init_waitqueue_head(&to_do.wait);
> +	init_waitqueue_head(&to_do.done);
> +	mutex_init(&to_do.lock);
>  
>   Unlock:
>  	mutex_unlock(&pm_mutex);
> @@ -131,6 +224,10 @@ static int snapshot_release(struct inode *inode, struct file *filp)
>  
>  	mutex_lock(&pm_mutex);
>  
> +	set_bit(TODO_CLOSED, to_do.flags);
> +	wake_up(&to_do.done);
> +	flush_workqueue(to_do.worker);
> +
>  	swsusp_free();
>  	free_basic_memory_bitmaps();
>  	data = filp->private_data;
> @@ -139,6 +236,7 @@ static int snapshot_release(struct inode *inode, struct file *filp)
>  		thaw_processes();
>  	pm_notifier_call_chain(data->mode == O_WRONLY ?
>  			PM_POST_HIBERNATION : PM_POST_RESTORE);
> +	hibernate_io_ops = data->prev_ops;
>  	atomic_inc(&snapshot_device_available);
>  
>  	mutex_unlock(&pm_mutex);
> @@ -152,6 +250,7 @@ static ssize_t snapshot_read(struct file *filp, char __user *buf,
>  	struct snapshot_data *data;
>  	ssize_t res;
>  	loff_t pg_offp = *offp & ~PAGE_MASK;
> +	int fin = 0;
>  
>  	mutex_lock(&pm_mutex);
>  
> @@ -161,18 +260,31 @@ static ssize_t snapshot_read(struct file *filp, char __user *buf,
>  		goto Unlock;
>  	}
>  	if (!pg_offp) { /* on page boundary? */
> -		res = snapshot_read_next(&data->handle);

I really don't understand why you're willing to do this.  In the s2disk case we
have an image in memory and we only want to trasfer it to user space
page-by-page, where user space decides when each page is going to be
transferred.  snapshot_read_next() is for that and while you can modify it
however you like, I don't think it's really worth it.

> -		if (res <= 0)
> +		res = wait_event_interruptible(to_do.wait,
> +			test_bit(TODO_WORK, to_do.flags) ||
> +			(fin = test_and_clear_bit(TODO_FINISH, to_do.flags)));
> +		if (res)
>  			goto Unlock;
> -	} else {
> -		res = PAGE_SIZE - pg_offp;
> +		if (test_bit(TODO_ERROR, to_do.flags)) {
> +			res = -EIO;
> +			goto Unlock;
> +		}
> +		if (fin)
> +			goto wake;
>  	}
> +	res = PAGE_SIZE - pg_offp;
>  
> -	res = simple_read_from_buffer(buf, count, &pg_offp,
> -			data_of(data->handle), res);
> +	mutex_lock(&to_do.lock);
> +	res = simple_read_from_buffer(buf, count, &pg_offp, to_do.buffer, res);
> +	mutex_unlock(&to_do.lock);
>  	if (res > 0)
>  		*offp += res;
>  
> +	if (!(pg_offp & ~PAGE_MASK)) {
> +		clear_bit(TODO_WORK, to_do.flags);
> +wake:
> +		wake_up(&to_do.done);
> +	}
>   Unlock:
>  	mutex_unlock(&pm_mutex);
>  
> @@ -278,8 +390,11 @@ static long snapshot_ioctl(struct file *filp, unsigned int cmd,
>  		error = hibernation_snapshot(data->platform_support);
>  		if (!error)
>  			error = put_user(in_suspend, (int __user *)arg);
> -		if (!error)
> +		if (!error) {
> +			if (in_suspend)
> +				queue_work(to_do.worker, &snapshot_writer_w);
>  			data->ready = 1;
> +		}
>  		break;
>  
>  	case SNAPSHOT_ATOMIC_RESTORE:
> @@ -473,7 +588,16 @@ static struct miscdevice snapshot_device = {
>  
>  static int __init snapshot_device_init(void)
>  {
> -	return misc_register(&snapshot_device);
> +	int ret;
> +
> +	to_do.worker = create_singlethread_workqueue("suspend_worker");
> +	if (!to_do.worker)
> +		return -ENOMEM;
> +
> +	ret = misc_register(&snapshot_device);
> +	if (ret)
> +		destroy_workqueue(to_do.worker);
> +	return ret;
>  };
>  
>  device_initcall(snapshot_device_init);

And using a special workqueue for this purpose is seriously over the top IMnshO.

Why don't you just regard s2disk as a special case and don't touch it
(or modify it only so much to prevent it from getting in the way)?

Rafael
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