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 15:25:26 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Oscar Salvador <osalvador@...e.de>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: 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 2/12/24 23:30, Oscar Salvador wrote:
> This patch adds a new directory called 'page_owner_stacks' under
> /sys/kernel/debug/, with a file called 'show_stacks' in it.
> Reading from that file will show all stacks that were added by page_owner
> followed by their counting, giving us a clear overview of stack <-> count
> relationship.
> 
> E.g:
> 
>   prep_new_page+0xa9/0x120
>   get_page_from_freelist+0x801/0x2210
>   __alloc_pages+0x18b/0x350
>   alloc_pages_mpol+0x91/0x1f0
>   folio_alloc+0x14/0x50
>   filemap_alloc_folio+0xb2/0x100
>   __filemap_get_folio+0x14a/0x490
>   ext4_write_begin+0xbd/0x4b0 [ext4]
>   generic_perform_write+0xc1/0x1e0
>   ext4_buffered_write_iter+0x68/0xe0 [ext4]
>   ext4_file_write_iter+0x70/0x740 [ext4]
>   vfs_write+0x33d/0x420
>   ksys_write+0xa5/0xe0
>   do_syscall_64+0x80/0x160
>   entry_SYSCALL_64_after_hwframe+0x6e/0x76
>  stack_count: 4578
> 
> The seq stack_{start,next} functions will iterate through the list
> stack_list in order to print all stacks.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>

..

> +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().

> +
> +	return 0;
> +}
> +