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

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Message-ID: <3c69f09e-1e34-73f9-b38b-733971ef38cb@gnuweeb.org>
Date:   Thu, 1 Sep 2022 15:31:51 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM 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>
Subject: Re: [PATCH 3/3] mm,page_owner: Filter out stacks by a threshold
 counter

On 9/1/22 11:42 AM, Oscar Salvador wrote:> +static ssize_t read_page_owner_threshold(struct file *file, char __user *buf,
> +					 size_t count, loff_t *pos)
> +{
> +	char *kbuf;
> +	int ret = 0;
> +
> +	count = min_t(size_t, count, PAGE_SIZE);
> +
> +	if (*pos >= count)
> +		return 0;
> +
> +	kbuf = kmalloc(count, GFP_KERNEL);
> +	if (!kbuf)
> +		return ENOMEM;

Missing a negative sign, return -ENOMEM.

> +	ret = scnprintf(kbuf, count, "%lu\n", threshold_count);
> +	if (copy_to_user(buf, kbuf, ret))
> +		ret = -EFAULT;
> +
> +	*pos += count;
> +	kfree(kbuf);
> +
> +	return ret;
> +}
> +
> +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);

This looks overestimating to me. For unsigned long, on a 64-bit system
has max val 18446744073709551615 (20 chars).

You can use stack a allocated local variable with length 21. No need
to use kmalloc(). The same way with the read() op.

> +	if (!kbuf)
> +		return -ENOMEM;
> +
> +	if (copy_from_user(kbuf, buf, count)) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +
> +	kbuf[count - 1] = '\0';
> +
> +	ret = kstrtoul(kbuf, 10, &threshold_count);
> +
> +out:
> +	kfree(kbuf);
> +	return ret ? ret : count;
> +}
> +
> +static const struct file_operations proc_page_owner_threshold = {
> +	.read = read_page_owner_threshold,
> +	.write = write_page_owner_threshold,
> +};

-- 
Ammar Faizi