Re: [PATCH v4 1/2] lib: stackdepot: Add support to configure STACK_HASH_SIZE
From: Vijayanand Jitta
Date: Mon Jan 18 2021 - 23:13:01 EST
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@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
>> - 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
I have updated the patch avoiding __initdata as per suggestion and the
copy from tmp , can you please review v5.
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