Re: [PATCH v4 1/2] lib: stackdepot: Add support to configure STACK_HASH_SIZE

[Vijayanand Jitta]

On 1/5/2021 4:42 AM, Andrew Morton wrote:
> On Wed, 30 Dec 2020 18:15:30 +0530 vjitta@xxxxxxxxxxxxxx wrote:
> 
>> Use STACK_HASH_ORDER_SHIFT to configure STACK_HASH_SIZE.
>>
>> Aim is to have configurable value for  STACK_HASH_SIZE,
>> so depend on use case one can configure it.
>>
>> One example is of Page Owner, default value of
>> STACK_HASH_SIZE lead stack depot to consume 8MB of static memory.
>> Making it configurable and use lower value helps to enable features like
>> CONFIG_PAGE_OWNER without any significant overhead.
> 
> Questions regarding the stackdepot code.
> 
> - stack_table_tmp[] is __initdata.  So after initmem is released,
>   that "consume 8MB of static memory" should no longer be true.  But
>   iirc, not all architectures actually release __initdata memory.  Does
>   your architecture do this?
> 
Thanks for review comments, I wasn't aware that __initdata is
architecture dependent, I was assuming that __initdata always frees
memory and yes the architecture which i am using (arm64) does free
__inidata.

> - Stackdepot copies stack_table_tmp[] into vmalloced memory during
>   initcalls.  Why?  Why not simply make stack_table_tmp[] no longer
>   __initdata and use that memory for all time?
> 
>   Presumably because in the stack_depot_disable==true case, we
>   release stack_table_tmp[] memory, don't vmalloc for a copy of it, and
>   save a bunch of memory?  If so, this assumes that the __initdata
>   memory is freed.
> 

Yes, that correct. assumption here is __initidata will free memory if
stack_depot_disable=true is set.

> - Why is that hash table so large?  Is it appropriately sized?
> 

I think the large size of hash table is justified since the users of
stack depot like kasan, page owner etc store a very large number of  stacks.

> - SMP is up and running during init_stackdepot(), I think?  If so, is
>   that huge memcpy smp-safe?  Can other CPUs be modifying
>   stack_table_tmp[] while the memcpy is in flight?
> 
> 
> 
Yes, parallel access could be possible. I will add a locking mechanism
inplace.

Thanks,
Vijay

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