Re: [PATCH] mm/slub: initialize stack depot in boot process

[Marco Elver <elver@google.com>]

On Mon, Feb 28, 2022 at 03:09PM +0000, Hyeonggon Yoo wrote:
> commit ba10d4b46655 ("mm/slub: use stackdepot to save stack trace in
> objects") initializes stack depot while creating cache if SLAB_STORE_USER
> flag is set.
> 
> This can make kernel crash because a cache can be created in various
> contexts. For example if user sets slub_debug=U, kernel crashes
> because create_boot_cache() calls stack_depot_init(), which tries to
> allocate hash table using memblock_alloc() if slab is not available.
> But memblock is also not available at that time.
> 
> This patch solves the problem by initializing stack depot early
> in boot process if SLAB_STORE_USER debug flag is set globally
> or the flag is set to at least one cache.
> 
> [ elver@google.com: initialize stack depot depending on slub_debug
>   parameter instead of allowing stack_depot_init() can be called
>   in kmem_cache_init() for simplicity. ]
> 
> Link: https://lkml.org/lkml/2022/2/28/238

This would be a better permalink:
https://lore.kernel.org/all/YhyeaP8lrzKgKm5A@ip-172-31-19-208.ap-northeast-1.compute.internal/

> Fixes: ba10d4b46655 ("mm/slub: use stackdepot to save stack trace in objects")

This commit does not exist in -next.

I assume you intend that "lib/stackdepot: Use page allocator if both
slab and memblock is unavailable" should be dropped now.

> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> ---
>  include/linux/slab.h |  1 +
>  init/main.c          |  1 +
>  mm/slab.c            |  4 ++++
>  mm/slob.c            |  4 ++++
>  mm/slub.c            | 28 +++++++++++++++++++++++++---
>  5 files changed, 35 insertions(+), 3 deletions(-)
[...]
>  
> +/* Initialize stack depot if needed */
> +void __init kmem_cache_init_early(void)
> +{
> +#ifdef CONFIG_STACKDEPOT
> +	slab_flags_t block_flags;
> +	char *next_block;
> +	char *slab_list;
> +
> +	if (slub_debug & SLAB_STORE_USER)
> +		goto init_stack_depot;
> +
> +	next_block = slub_debug_string;
> +	while (next_block) {
> +		next_block = parse_slub_debug_flags(next_block, &block_flags, &slab_list, false);
> +		if (block_flags & SLAB_STORE_USER)
> +			goto init_stack_depot;
> +	}
> +
> +	return;
> +
> +init_stack_depot:
> +	stack_depot_init();
> +#endif
> +}

You can simplify this function to avoid the goto:

	/* Initialize stack depot if needed */
	void __init kmem_cache_init_early(void)
	{
	#ifdef CONFIG_STACKDEPOT
		slab_flags_t flags = slub_debug;
		char *next_block = slub_debug_string;
		char *slab_list;
	
		for (;;) {
			if (flags & SLAB_STORE_USER) {
				stack_depot_init();
				break;
			}
			if (!next_block)
				break;
			next_block = parse_slub_debug_flags(next_block, &flags, &slab_list, false);
		}
	#endif
	}

^^ with this version, it'd also be much easier and less confusing to add
other initialization logic unrelated to stackdepot later after the loop
(should it ever be required).