linux-kernel - Re: [PATCH v2 2/3] mm, page_owner: Add page_owner_stacks file to print out only stacks and their counter

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Message-ID: <YxXyThZanSl3wboo@elver.google.com>
Date:   Mon, 5 Sep 2022 14:57:50 +0200
From:   Marco Elver <elver@...gle.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, 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>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v2 2/3] mm, page_owner: Add page_owner_stacks file to
 print out only stacks and their counter

On Mon, Sep 05, 2022 at 05:10AM +0200, Oscar Salvador wrote:
[...]
> +int stack_depot_print_stacks_threshold(char *buf, size_t size, loff_t *pos)

Can you add kernel-doc comment what this does (and also update
accordingly in 3/3 when you add 'threshold').

>From what I see it prints *all* stacks that have a non-zero count.
Correct?

If so, should this be called stack_depot_print_all_count() (having
stack(s) in the name twice doesn't make it more obvious what it does)?
Then in the follow-up patch you add the 'threshold' arg.

> +{
> +	int i = *pos, ret = 0;
> +	struct stack_record **stacks, *stack;
> +	static struct stack_record *last = NULL;
> +	unsigned long stack_table_entries = stack_hash_mask + 1;
> +
> +	/* Continue from the last stack if we have one */
> +	if (last) {
> +		stack = last->next;

This is dead code?

> +	} else {
> +new_table:
> +		stacks = &stack_table[i];
> +		stack = (struct stack_record *)stacks;
> +	}
> +
> +	for (; stack; stack = stack->next) {
> +		if (!stack->size || stack->size < 0 ||
> +		    stack->size > size || stack->handle.valid != 1 ||
> +		    refcount_read(&stack->count) < 1)
> +			continue;
> +
> +		ret += stack_trace_snprint(buf, size, stack->entries, stack->size, 0);
> +		ret += scnprintf(buf + ret, size - ret, "stack count: %d\n\n",
> +				 refcount_read(&stack->count));
> +		last = stack;
> +		return ret;
> +	}
> +
> +	i++;
> +	*pos = i;
> +	last = NULL;
> +
> +	/* Keep looking all tables for valid stacks */
> +	if (i < stack_table_entries)
> +		goto new_table;
> +
> +	return 0;
> +}

Either I'm missing something really obvious, but I was able to simplify
the above function to just this (untested!):

	int stack_depot_print_stacks_threshold(char *buf, size_t size, loff_t *pos)
	{
		const unsigned long stack_table_entries = stack_hash_mask + 1;

		/* Iterate over all tables for valid stacks. */
		for (; *pos < stack_table_entries; (*pos)++) {
			for (struct stack_record *stack = stack_table[*pos]; stack; stack = stack->next) {
				if (!stack->size || stack->size < 0 || stack->size > size ||
				    stack->handle.valid != 1 || refcount_read(&stack->count) < 1)
					continue;

				return stack_trace_snprint(buf, size, stack->entries, stack->size, 0) +
				       scnprintf(buf + ret, size - ret, "stack count: %d\n\n",
						 refcount_read(&stack->count));
			}
		}

		return 0;
	}

> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 8730f377fa91..d88e6b4aefa0 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -664,6 +664,29 @@ static void init_early_allocated_pages(void)
>  		init_zones_in_node(pgdat);
>  }
>  
> +static ssize_t read_page_owner_stacks(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);
> +	kbuf = kmalloc(count, GFP_KERNEL);
> +	if (!kbuf)
> +		return -ENOMEM;
> +
> +	ret += stack_depot_print_stacks_threshold(kbuf, count, pos);

If I understood right, this will print *all* stacks that have non-zero
count, and this isn't related to page_owner per-se. Correct?

This might not be a problem right now, but once there might be more
users that want to count stack usage, you'll end up with page_owner +
other stacks here.