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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/28/22 16:09, 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
> Fixes: ba10d4b46655 ("mm/slub: use stackdepot to save stack trace in objects")
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

I think a much easier approach would be to do this checking in
setup_slub_debug(). There we may either detect SLAB_STORE_USER in
global_flags, or check the flags returned by parse_slub_debug_flags() in the
while (str) cycle, in the 'else' case where slab_list is present. Both cases
would just set some variable that stack_depot_early_init() (the
!CONFIG_STACKDEPOT_ALWAYS_INIT version, or a newly consolidated one) would
check. So that would be another way to request the stack_depot_init() at a
well-defined point of boot, similar to CONFIG_STACKDEPOT_ALWAYS_INIT.
Because setup_slub_debug() is called by __setup, which is processed from
start_kernel() -> parse_args() before mm_init() -> stack_depot_early_init().

> ---
>  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(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 37bde99b74af..023f3f71ae35 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -139,6 +139,7 @@ struct mem_cgroup;
>  /*
>   * struct kmem_cache related prototypes
>   */
> +void __init kmem_cache_init_early(void);
>  void __init kmem_cache_init(void);
>  bool slab_is_available(void);
>  
> diff --git a/init/main.c b/init/main.c
> index 65fa2e41a9c0..4fdb7975a085 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -835,6 +835,7 @@ static void __init mm_init(void)
>  	kfence_alloc_pool();
>  	report_meminit();
>  	stack_depot_early_init();
> +	kmem_cache_init_early();
>  	mem_init();
>  	mem_init_print_info();
>  	kmem_cache_init();
> diff --git a/mm/slab.c b/mm/slab.c
> index ddf5737c63d9..80a6d01aab06 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1196,6 +1196,10 @@ static void __init set_up_node(struct kmem_cache *cachep, int index)
>  	}
>  }
>  
> +void __init kmem_cache_init_early(void)
> +{
> +}
> +
>  /*
>   * Initialisation.  Called after the page allocator have been initialised and
>   * before smp_init().
> diff --git a/mm/slob.c b/mm/slob.c
> index 60c5842215f1..00e323af8be4 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -715,6 +715,10 @@ struct kmem_cache kmem_cache_boot = {
>  	.align = ARCH_KMALLOC_MINALIGN,
>  };
>  
> +void __init kmem_cache_init_early(void)
> +{
> +}
> +
>  void __init kmem_cache_init(void)
>  {
>  	kmem_cache = &kmem_cache_boot;
> diff --git a/mm/slub.c b/mm/slub.c
> index a74afe59a403..40bcd18143b6 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4221,9 +4221,6 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
>  	s->remote_node_defrag_ratio = 1000;
>  #endif
>  
> -	if (s->flags & SLAB_STORE_USER && IS_ENABLED(CONFIG_STACKDEPOT))
> -		stack_depot_init();
> -
>  	/* Initialize the pre-computed randomized freelist if slab is up */
>  	if (slab_state >= UP) {
>  		if (init_cache_random_seq(s))
> @@ -4810,6 +4807,31 @@ static struct kmem_cache * __init bootstrap(struct kmem_cache *static_cache)
>  	return s;
>  }
>  
> +/* 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
> +}
> +
>  void __init kmem_cache_init(void)
>  {
>  	static __initdata struct kmem_cache boot_kmem_cache,