linux-kernel - Re: [RFC 1/3] /dev/low_mem

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Date:	Tue, 24 Jan 2012 14:57:13 -0700
From:	Jonathan Corbet <corbet@....net>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Rik van Riel <riel@...hat.com>, Minchan Kim <minchan@...nel.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, leonid.moiseichuk@...ia.com,
	kamezawa.hiroyu@...fujitsu.com, mel@....ul.ie, rientjes@...gle.com,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ronen Hod <rhod@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC 1/3] /dev/low_mem_notify

On Tue, 17 Jan 2012 20:51:13 +0200 (EET)
Pekka Enberg <penberg@...nel.org> wrote:

> Ok, so here's a proof of concept patch that implements sample-base 
> per-process free threshold VM event watching using perf-like syscall ABI. 
> I'd really like to see something like this that's much more extensible and 
> clean than the /dev based ABIs that people have proposed so far.

OK, so I'm slow, but better late than never.  I plead travel.

I guess the thing that surprises me is that nobody has said this yet: this
looks a lot like an event-reporting mechanism like perf.  Is there a reason
these can't be perf-style events integrated with all the rest?

> +struct vmnotify_config {
> +	/*
> +	 * Size of the struct for ABI extensibility.
> +	 */
> +	__u32		   size;
> +
> +	/*
> +	 * Notification type bitmask
> +	 */
> +	__u64			type;
> +
> +	/*
> +	 * Free memory threshold in percentages [1..99]
> +	 */
> +	__u32			free_threshold;

Is this an upper-bound threshold or a lower-bound threshold?  From your
example, it looks like "free_threshold" is "the amount of memory that is
not free", which seems confusing.

[...]

> new file mode 100644
> index 0000000..6800450
> --- /dev/null
> +++ b/mm/vmnotify.c
> @@ -0,0 +1,235 @@
> +#include <linux/anon_inodes.h>
> +#include <linux/vmnotify.h>
> +#include <linux/syscalls.h>
> +#include <linux/file.h>
> +#include <linux/list.h>
> +#include <linux/poll.h>
> +#include <linux/slab.h>
> +#include <linux/swap.h>
> +
> +#define VMNOTIFY_MAX_FREE_THRESHOD	100

Did we run out of L's here? :)

> +static ssize_t vmnotify_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
> +{
> +	struct vmnotify_watch *watch = file->private_data;
> +	int ret = 0;
> +
> +	mutex_lock(&watch->mutex);
> +
> +	if (!watch->pending)
> +		goto out_unlock;
> +
> +	if (copy_to_user(buf, &watch->event, sizeof(struct vmnotify_event))) {
> +		ret = -EFAULT;
> +		goto out_unlock;
> +	}
> +
> +	ret = watch->event.size;
> +
> +	watch->pending = false;
> +
> +out_unlock:
> +	mutex_unlock(&watch->mutex);
> +
> +	return ret;
> +}

So this is a nonblocking-only interface?  That may surprise some
developers.  You already have a wait queue, why not wait on it if need be?

> +static int vmnotify_copy_config(struct vmnotify_config __user *uconfig,
> +				struct vmnotify_config *config)
> +{
> +	int ret;
> +
> +	ret = copy_from_user(config, uconfig, sizeof(struct vmnotify_config));
> +	if (ret)
> +		return -EFAULT;
> +
> +	if (!config->type)
> +		return -EINVAL;
> +
> +	if (config->type & VMNOTIFY_TYPE_SAMPLE) {
> +		if (config->sample_period_ns < NSEC_PER_MSEC)
> +			return -EINVAL;
> +	}

What happens if the sample period is zero?

jon
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