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

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Date:   Mon, 5 Sep 2022 13:31:02 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management Mailing List <linux-mm@...ck.org>,
        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 05-09-22 17:51:37, Ammar Faizi wrote:
> 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;
> }

Isn't there a proc_dointvec counterpart for debugfs?

-- 
Michal Hocko
SUSE Labs