linux-kernel - Re: [PATCH] tools: adding support for idle page tracking to tool

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Date:   Tue, 19 Jun 2018 15:39:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Christian Hansen <chansen3@...co.com>
Cc:     xe-linux-external@...co.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tools: adding support for idle page tracking to tool

On Tue, 12 Jun 2018 11:32:23 -0400 Christian Hansen <chansen3@...co.com> wrote:

> Adding a flag which will use the kernels's idle
> page tracking to mark pages idle.  As the tool already
> prints the idle flag if set, subsequent runs will show
> which pages have been accessed since last run.

That sounds useful.

This patch seems to have been prepared against the mainline kernel, so
it conflicts with your "tools: modifying page-types to include shared
map counts" patch.  Awkward, but I seem to have got it fixed up.

> ...
>
> @@ -566,6 +570,30 @@ static int unpoison_page(unsigned long offset)
>  	return 0;
>  }
>  
> +static int mark_page_idle(unsigned long offset)
> +{
> +	static unsigned long off;
> +	static uint64_t buf;
> +	int len;
> +
> +	if ((offset / 64 == off / 64) || buf == 0) {
> +		buf |= 1UL << (offset % 64);
> +		off = offset;
> +		return 0;
> +	}
> +
> +	len = pwrite(page_idle_fd, &buf, 8, 8 * (off / 64));
> +	if (len < 0) {
> +		perror("mark page idle");
> +		return len;
> +	}
> +
> +	buf = 1UL << (offset % 64);
> +	off = offset;
> +
> +	return 0;
> +}

This is a bit cumbersome.  Why not this way:

static int mark_page_idle(unsigned long offset)
{
	static unsigned long off;
	static uint64_t buf;
	int len;

	if ((offset / 64 != off / 64) && buf != 0) {
		len = pwrite(page_idle_fd, &buf, 8, 8 * (off / 64));
		if (len < 0) {
			perror("mark page idle");
			return len;
		}
	}
	buf = 1UL << (offset % 64);
	off = offset;
	return 0;
}

Also, it's not very clear what's going on here - the handling of
offset, off and buf.  Some well-crafted comments would help.

>
> ...
>