Re: [PATCH v3 1/6] lib/stackdepot: allow requesting early initialization dynamically

[David Rientjes <rientjes@google.com>]

On Mon, 4 Apr 2022, 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 parameters (which happens early enough)
> can call a new function stack_depot_want_early_init(), which sets a flag
> that stack_depot_early_init() will check.
> 
> 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().
> 
> As suggested by Mike, make stack_depot_early_init() only attempt
> memblock allocation and stack_depot_init() only attempt kvmalloc().
> Also change the latter to kvcalloc(). In both cases we can lose the
> explicit array zeroing, which the allocations do already.
> 
> As suggested by Marco, provide empty implementations of the init
> functions for !CONFIG_STACKDEPOT builds to simplify the callers.
> 
> [1] https://lore.kernel.org/all/YhnUcqyeMgCrWZbd@ip-172-31-19-208.ap-northeast-1.compute.internal/
> 
> Reported-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
> Suggested-by: Marco Elver <elver@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Reviewed-by: Marco Elver <elver@google.com>
> Reviewed-and-tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
>  include/linux/stackdepot.h | 26 ++++++++++++---
>  lib/stackdepot.c           | 66 +++++++++++++++++++++++++-------------
>  mm/page_owner.c            |  9 ++++--
>  3 files changed, 72 insertions(+), 29 deletions(-)
> 
> diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
> index 17f992fe6355..bc2797955de9 100644
> --- a/include/linux/stackdepot.h
> +++ b/include/linux/stackdepot.h
> @@ -20,18 +20,36 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  					gfp_t gfp_flags, bool can_alloc);
>  
>  /*
> - * Every user of stack depot has to call this during its own init when it's
> - * decided that it will be calling stack_depot_save() later.
> + * Every user of stack depot has to call stack_depot_init() during its own init
> + * when it's decided that it will be calling stack_depot_save() later. This is
> + * recommended for e.g. modules initialized later in the boot process, when
> + * slab_is_available() is true.
>   *
>   * 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 call stack_depot_want_early_init(), when the
> + * decision to use stack depot is taken e.g. when evaluating kernel boot
> + * parameters, which precedes the enablement point in mm_init().
> + *
> + * stack_depot_init() and stack_depot_want_early_init() can be called regardless
> + * of CONFIG_STACKDEPOT and are no-op when disabled. The actual save/fetch/print
> + * functions should only be called from code that makes sure CONFIG_STACKDEPOT
> + * is enabled.
>   */
> +#ifdef CONFIG_STACKDEPOT
>  int stack_depot_init(void);
>  
> -#ifdef CONFIG_STACKDEPOT_ALWAYS_INIT
> -static inline int stack_depot_early_init(void)	{ return stack_depot_init(); }
> +void __init stack_depot_want_early_init(void);
> +
> +/* This is supposed to be called only from mm_init() */
> +int __init stack_depot_early_init(void);
>  #else
> +static inline int stack_depot_init(void) { return 0; }
> +
> +static inline void stack_depot_want_early_init(void) { }
> +
>  static inline int stack_depot_early_init(void)	{ return 0; }
>  #endif
>  
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index bf5ba9af0500..6c4644c9ed44 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -66,6 +66,9 @@ struct stack_record {
>  	unsigned long entries[];	/* Variable-sized array of entries. */
>  };
>  
> +static bool __stack_depot_want_early_init __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
> +static bool __stack_depot_early_init_passed __initdata;
> +
>  static void *stack_slabs[STACK_ALLOC_MAX_SLABS];
>  
>  static int depot_index;
> @@ -162,38 +165,57 @@ static int __init is_stack_depot_disabled(char *str)
>  }
>  early_param("stack_depot_disable", is_stack_depot_disabled);
>  
> -/*
> - * __ref because of memblock_alloc(), which will not be actually called after
> - * the __init code is gone, because at that point slab_is_available() is true
> - */
> -__ref int stack_depot_init(void)
> +void __init stack_depot_want_early_init(void)
> +{
> +	/* Too late to request early init now */
> +	WARN_ON(__stack_depot_early_init_passed);
> +
> +	__stack_depot_want_early_init = true;
> +}
> +
> +int __init stack_depot_early_init(void)
> +{
> +	size_t size;
> +
> +	/* This is supposed to be called only once, from mm_init() */
> +	if (WARN_ON(__stack_depot_early_init_passed))
> +		return 0;
> +
> +	__stack_depot_early_init_passed = true;
> +
> +	if (!__stack_depot_want_early_init || stack_depot_disable)
> +		return 0;
> +
> +	pr_info("Stack Depot early init allocating hash table with memblock_alloc\n");
> +	size = (STACK_HASH_SIZE * sizeof(struct stack_record *));

I think the kvcalloc() in the main init path is very unlikely to fail, but 
perhaps this memblock_alloc() might?  If so, a nit might be to include 
this size as part of the printk.

Either way:

Acked-by: David Rientjes <rientjes@google.com>