linux-kernel - Re: [PATCH v8 3/5] mm,page_owner: Display all stacks and their count

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Date: Tue, 13 Feb 2024 16:33:30 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Michal Hocko <mhocko@...e.com>,
	Marco Elver <elver@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v8 3/5] mm,page_owner: Display all stacks and their count

On Tue, Feb 13, 2024 at 03:25:26PM +0100, Vlastimil Babka wrote:
> On 2/12/24 23:30, Oscar Salvador wrote:
> > +static int stack_print(struct seq_file *m, void *v)
> > +{
> > +	char *buf;
> > +	int ret = 0;
> > +	struct stack *stack = v;
> > +	struct stack_record *stack_record = stack->stack_record;
> > +
> > +	if (!stack_record->size || stack_record->size < 0 ||
> > +	    refcount_read(&stack_record->count) < 2)
> > +		return 0;
> > +
> > +	buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
> > +
> > +	ret += stack_trace_snprint(buf, PAGE_SIZE, stack_record->entries,
> > +				   stack_record->size, 0);
> > +	if (!ret)
> > +		goto out;
> > +
> > +	scnprintf(buf + ret, PAGE_SIZE - ret, "stack_count: %d\n\n",
> > +		  refcount_read(&stack_record->count));
> > +
> > +	seq_printf(m, buf);
> > +	seq_puts(m, "\n\n");
> > +out:
> > +	kfree(buf);
> 
> Seems rather wasteful to do kzalloc/kfree so you can print into that buffer
> first and then print/copy it again using seq_printf. If you give up on using
> stack_trace_snprintf() it's not much harder to print the stack directly with
> a loop of seq_printf. See e.g. slab_debugfs_show().

Well, I thought about not reinventing the wheel there, but fair enough
than performing a kmalloc/free op on every print might be suboptimal.
I will try to do ir with seq_printf alone.

Thanks
 

-- 
Oscar Salvador
SUSE Labs