linux-kernel - Re: [PATCH v2 3/3] mm,page_owner: Filter out stacks by a threshold counter

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Message-Id: <573c4049045a5ff342ff9998c69dfae7-ammarfaizi2@gnuweeb.org>
Date:   Mon,  5 Sep 2022 17:51:37 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management Mailing List <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Eric Dumazet <edumazet@...gle.com>,
        Waiman Long <longman@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v2 3/3] mm,page_owner: Filter out stacks by a threshold counter

On Mon, 5 Sep 2022 05:10:12 +0200, Oscar Salvador wrote:
> +static int page_owner_threshold_show(struct seq_file *p, void *v)
> +{
> +	 seq_printf(p, "%lu\n", threshold);

Remove a slipped leading 0x20 space here (before seq_printf()).

> +	return 0;
> +}
> +
> +static ssize_t write_page_owner_threshold(struct file *file, const char __user *buf,
> +					  size_t count, loff_t *pos)
> +{
> +	char *kbuf;
> +	int ret = 0;
> +
> +	count = min_t(size_t, count, PAGE_SIZE);
> +	kbuf = kmalloc(count, GFP_KERNEL);
> +	if (!kbuf)
> +		return -ENOMEM;
> +
> +	if (copy_from_user(kbuf, buf, count)) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +
> +	kbuf[count - 1] = '\0';
> +
> +	ret = kstrtoul(kbuf, 10, &threshold);
> +
> +out:
> +	kfree(kbuf);
> +	return ret ? ret : count;
> +}

Still the same comment on this, kmalloc() is not really needed here.
Capping the size to PAGE_SIZE (usually 4K) is too big. `unsinged long`
is 64-bit at most, this means the max val is 18446744073709551615
(20 chars). The lifetime of @kbuf is very short as well, using a stack
allocated array of chars is fine?

Untested:

static ssize_t write_page_owner_threshold(struct file *file, const char __user *buf,
					  size_t count, loff_t *pos)
{
	char kbuf[21];
	int ret;

	count = min_t(size_t, count, sizeof(kbuf));
	if (copy_from_user(kbuf, buf, count))
		return -EFAULT;

	kbuf[count - 1] = '\0';
	ret = kstrtoul(kbuf, 10, &threshold);
	return ret ? ret : count;
}

-- 
Ammar Faizi