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Subject: Re: [PATCH v4 1/2] lib: stackdepot: Add support to configure
 STACK_HASH_SIZE
From:   Vijayanand Jitta <vjitta@codeaurora.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     minchan@kernel.org, glider@google.com, dan.j.williams@intel.com,
        broonie@kernel.org, mhiramat@kernel.org,
        linux-kernel@vger.kernel.org, ylal@codeaurora.org,
        vinmenon@codeaurora.org, Thomas Gleixner <tglx@linutronix.de>
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 <20210104151223.34f97a033e966c9cc89915cb@linux-foundation.org>
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Date:   Tue, 19 Jan 2021 09:32:27 +0530
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On 1/5/2021 2:54 PM, Vijayanand Jitta wrote:
> 
> 
> On 1/5/2021 4:42 AM, Andrew Morton wrote:
>> On Wed, 30 Dec 2020 18:15:30 +0530 vjitta@codeaurora.org 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
> 

I have updated the patch avoiding __initdata as per suggestion and the
copy from tmp , can you please review v5.

https://lore.kernel.org/patchwork/patch/1367306/

Thanks,
Vijay

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