Re: [PATCH v2 1/6] lib/stackdepot: allow requesting early initialization dynamically

[Marco Elver]

On Wed, Mar 02, 2022 at 06:31PM +0100, Vlastimil Babka wrote:
> In a later patch we want to add stackdepot support for object owner
> tracking in slub caches, which is enabled by slub_debug boot parameter.
> This creates a bootstrap problem as some caches are created early in
> boot when slab_is_available() is false and thus stack_depot_init()
> tries to use memblock. But, as reported by Hyeonggon Yoo [1] we are
> already beyond memblock_free_all(). Ideally memblock allocation should
> fail, yet it succeeds, but later the system crashes, which is a
> separately handled issue.
> 
> To resolve this boostrap issue in a robust way, this patch adds another
> way to request stack_depot_early_init(), which happens at a well-defined
> point of time. In addition to build-time CONFIG_STACKDEPOT_ALWAYS_INIT,
> code that's e.g. processing boot parmeters (which happens early enough)
> can set a new variable stack_depot_want_early_init as true.

Agree, I think this is the best solution.

> In this patch we also convert page_owner to this approach. While it
> doesn't have the bootstrap issue as slub, it's also a functionality
> enabled by a boot param and can thus request stack_depot_early_init()
> with memblock allocation instead of later initialization with
> kvmalloc().
> 
> [1] https://lore.kernel.org/all/YhnUcqyeMgCrWZbd@xxxxxxxxxxxxxxxxxxx-northeast-1.compute.internal/
> 
> Reported-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  include/linux/stackdepot.h | 16 ++++++++++++++--
>  lib/stackdepot.c           |  2 ++
>  mm/page_owner.c            |  9 ++++++---
>  3 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
> index 17f992fe6355..1217ba2b636e 100644
> --- a/include/linux/stackdepot.h
> +++ b/include/linux/stackdepot.h
> @@ -15,6 +15,8 @@
>  
>  typedef u32 depot_stack_handle_t;
>  
> +extern bool stack_depot_want_early_init;
> +
>  depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  					unsigned int nr_entries,
>  					gfp_t gfp_flags, bool can_alloc);
> @@ -26,11 +28,21 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>   * The alternative is to select STACKDEPOT_ALWAYS_INIT to have stack depot
>   * enabled as part of mm_init(), for subsystems where it's known at compile time
>   * that stack depot will be used.
> + *
> + * Another alternative is to set stack_depot_want_early_init as true, when the
> + * decision to use stack depot is taken e.g. when evaluating kernel boot
> + * parameters, which precedes the call to stack_depot_want_early_init().
>   */
>  int stack_depot_init(void);

I think for stack_depot_init() it might now be convenient to provide a
no-op version automatically if !STACKDEPOT, which would avoid
some 'if (.. && IS_ENABLED(CONFIG_STACKDEPOT))' in a later patch.

Similarly, for stack_depot_want_early_init, where instead you could
simply provide stack_depot_want_early_init() as a function, which simply
sets a boolean __stack_depot_want_early_init. If !STACKDEPOT, it'll also
just be a no-op function.

>  
> -#ifdef CONFIG_STACKDEPOT_ALWAYS_INIT
> -static inline int stack_depot_early_init(void)	{ return stack_depot_init(); }
> +#ifdef CONFIG_STACKDEPOT
> +static inline int stack_depot_early_init(void)
> +{
> +	if (IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT)
> +	    || stack_depot_want_early_init)
> +		return stack_depot_init();
> +	return 0;
> +}
>  #else
>  static inline int stack_depot_early_init(void)	{ return 0; }
>  #endif
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index bf5ba9af0500..02e2b5fcbf3b 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -66,6 +66,8 @@ struct stack_record {
>  	unsigned long entries[];	/* Variable-sized array of entries. */
>  };
>  
> +bool stack_depot_want_early_init = false;
> +

This can be __initdata, right?

>  static void *stack_slabs[STACK_ALLOC_MAX_SLABS];
>  
>  static int depot_index;
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 99e360df9465..40dce2b81d13 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -42,7 +42,12 @@ static void init_early_allocated_pages(void);
>  
>  static int __init early_page_owner_param(char *buf)
>  {
> -	return kstrtobool(buf, &page_owner_enabled);
> +	int ret = kstrtobool(buf, &page_owner_enabled);
> +
> +	if (page_owner_enabled)
> +		stack_depot_want_early_init = true;
> +
> +	return ret;
>  }
>  early_param("page_owner", early_page_owner_param);
>  
> @@ -80,8 +85,6 @@ static __init void init_page_owner(void)
>  	if (!page_owner_enabled)
>  		return;
>  
> -	stack_depot_init();
> -
>  	register_dummy_stack();
>  	register_failure_stack();
>  	register_early_stack();
> -- 
> 2.35.1
>