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From:   Ammar Faizi <ammarfaizi2@gnuweeb.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Ammar Faizi <ammarfaizi2@gnuweeb.org>,
        Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Memory Management Mailing List <linux-mm@kvack.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Eric Dumazet <edumazet@google.com>,
        Waiman Long <longman@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Marco Elver <elver@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Alexander Potapenko <glider@google.com>
Subject: Re: [PATCH v2 3/3] mm,page_owner: Filter out stacks by a threshold counter
Date:   Mon,  5 Sep 2022 18:54:59 +0700
Message-Id: <20220905115459.567583-1-ammarfaizi2@gnuweeb.org>
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On Mon, 5 Sep 2022 13:31:02 +0200, Michal Hocko wrote:
> 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?

Ah, well. If that's much simpler, we should go with that. I am not
familiar proc_dointvec() interface, so I couldn't say about it.

Thanks for the comment. TIL.

-- 
Ammar Faizi