Re: [RFC 1/3] mm: Add VM pressure notifications

[Jonathan Corbet <corbet@lwn.net>]

On Wed, 7 Nov 2012 03:01:28 -0800
Anton Vorontsov <anton.vorontsov@linaro.org> wrote:

> This patch introduces vmpressure_fd() system call. The system call creates
> a new file descriptor that can be used to monitor Linux' virtual memory
> management pressure.

I noticed a couple of quick things as I was looking this over...

> +static ssize_t vmpressure_read(struct file *file, char __user *buf,
> +			       size_t count, loff_t *ppos)
> +{
> +	struct vmpressure_watch *watch = file->private_data;
> +	struct vmpressure_event event;
> +	int ret;
> +
> +	if (count < sizeof(event))
> +		return -EINVAL;
> +
> +	ret = wait_event_interruptible(watch->waitq,
> +				       atomic_read(&watch->pending));

Would it make sense to support non-blocking reads?  Perhaps a process would
like to simply know that current pressure level?

> +SYSCALL_DEFINE1(vmpressure_fd, struct vmpressure_config __user *, config)
> +{
> +	struct vmpressure_watch *watch;
> +	struct file *file;
> +	int ret;
> +	int fd;
> +
> +	watch = kzalloc(sizeof(*watch), GFP_KERNEL);
> +	if (!watch)
> +		return -ENOMEM;
> +
> +	ret = copy_from_user(&watch->config, config, sizeof(*config));
> +	if (ret)
> +		goto err_free;

This is wrong - you'll return the number of uncopied bytes to user space.
You'll need a "ret = -EFAULT;" in there somewhere.

jon