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

[Vlastimil Babka]

On 3/2/22 18:47, Marco Elver wrote:
> 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.

Thanks.

>> 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.

Yeah, makes sense. I guess I have patch 3/6 wrong now anyway as with
!STACKDEPOT it should fail linking due to missing stack_depot_want_early_init...

>>  
>> -#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?
 
I initially thought so too, but in include/linux/init.h found
 * Don't forget to initialize data not at file scope, i.e. within a function,
 * as gcc otherwise puts the data into the bss section and not into the init
 * section.
But maybe that's just outdated as everyone seems to init them at file scope.

>>  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
>>