linux-kernel - Re: [PATCH -mm v9 4/8] proc: add kpagecgroup file

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Date:	Tue, 21 Jul 2015 16:34:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jonathan Corbet <corbet@....net>, <linux-api@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v9 4/8] proc: add kpagecgroup file

On Sun, 19 Jul 2015 15:31:13 +0300 Vladimir Davydov <vdavydov@...allels.com> wrote:

> /proc/kpagecgroup contains a 64-bit inode number of the memory cgroup
> each page is charged to, indexed by PFN. Having this information is
> useful for estimating a cgroup working set size.
> 
> The file is present if CONFIG_PROC_PAGE_MONITOR && CONFIG_MEMCG.
>
> ...
>
> @@ -225,10 +226,62 @@ static const struct file_operations proc_kpageflags_operations = {
>  	.read = kpageflags_read,
>  };
>  
> +#ifdef CONFIG_MEMCG
> +static ssize_t kpagecgroup_read(struct file *file, char __user *buf,
> +				size_t count, loff_t *ppos)
> +{
> +	u64 __user *out = (u64 __user *)buf;
> +	struct page *ppage;
> +	unsigned long src = *ppos;
> +	unsigned long pfn;
> +	ssize_t ret = 0;
> +	u64 ino;
> +
> +	pfn = src / KPMSIZE;
> +	count = min_t(unsigned long, count, (max_pfn * KPMSIZE) - src);
> +	if (src & KPMMASK || count & KPMMASK)
> +		return -EINVAL;

The user-facing documentation should explain that reads must be
performed in multiple-of-8 sizes.

> +	while (count > 0) {
> +		if (pfn_valid(pfn))
> +			ppage = pfn_to_page(pfn);
> +		else
> +			ppage = NULL;
> +
> +		if (ppage)
> +			ino = page_cgroup_ino(ppage);
> +		else
> +			ino = 0;
> +
> +		if (put_user(ino, out)) {
> +			ret = -EFAULT;

Here we do the usual procfs violation of read() behaviour.  read()
normally only returns an error if it read nothing.  This code will
transfer a megabyte then return -EFAULT so userspace doesn't know that
it got that megabyte.

That's easy to fix, but procfs files do this all over the place anyway :(

> +			break;
> +		}
> +
> +		pfn++;
> +		out++;
> +		count -= KPMSIZE;
> +	}
> +
> +	*ppos += (char __user *)out - buf;
> +	if (!ret)
> +		ret = (char __user *)out - buf;
> +	return ret;
> +}
> +

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