Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH_SIZE

[Vijayanand Jitta <vjitta@codeaurora.org>]

On 12/14/2020 4:02 PM, Vijayanand Jitta wrote:
> 
> 
> On 12/14/2020 3:04 PM, Alexander Potapenko wrote:
>> On Mon, Dec 14, 2020 at 5:02 AM Vijayanand Jitta <vjitta@codeaurora.org> wrote:
>>>
>>>
>>>
>>> On 12/11/2020 6:55 PM, Alexander Potapenko wrote:
>>>> On Fri, Dec 11, 2020 at 1:45 PM Vijayanand Jitta <vjitta@codeaurora.org> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 12/11/2020 2:06 PM, Alexander Potapenko wrote:
>>>>>> On Thu, Dec 10, 2020 at 6:01 AM <vjitta@codeaurora.org> wrote:
>>>>>>>
>>>>>>> From: Yogesh Lal <ylal@codeaurora.org>
>>>>>>>
>>>>>>> Add a kernel parameter stack_hash_order to configure STACK_HASH_SIZE.
>>>>>>>
>>>>>>> Aim is to have configurable value for STACK_HASH_SIZE, so that one
>>>>>>> can configure it depending on usecase there by reducing the static
>>>>>>> memory overhead.
>>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> Can we go with a static CONFIG_ parameter instead?
>>>>>> Guess most users won't bother changing the default anyway, and for
>>>>>> CONFIG_PAGE_OWNER users changing the size at boot time is not strictly
>>>>>> needed.
>>>>>>
>>>>> Thanks for review.
>>>>>
>>>>> One advantage of having run time parameter is we can simply set it to a
>>>>> lower value at runtime if page_owner=off thereby reducing the memory
>>>>> usage or use default value if we want to use page owner so, we have some
>>>>> some flexibility here. This is not possible with static parameter as we
>>>>> have to have some predefined value.
>>>>
>>>> If we are talking about a configuration in which page_owner is the
>>>> only stackdepot user in the system, then for page_owner=off it
>>>> probably makes more sense to disable stackdepot completely instead of
>>>> setting it to a lower value. This is a lot easier to do in terms of
>>>> correctness.
>>>> But if there are other users (e.g. KASAN), their stackdepot usage may
>>>> actually dominate that of page_owner.
>>>>
>>>>>>> -static struct stack_record *stack_table[STACK_HASH_SIZE] = {
>>>>>>> -       [0 ...  STACK_HASH_SIZE - 1] = NULL
>>>>>>> +static unsigned int stack_hash_order = 20;
>>>>>>
>>>>>> Please initialize with MAX_STACK_HASH_ORDER instead.
>>>>>>
>>>>>
>>>>> Sure, will update this.
>>>>>
>>>>
>>>>
>>>>>>> +
>>>>>>> +static int __init init_stackdepot(void)
>>>>>>> +{
>>>>>>> +       size_t size = (STACK_HASH_SIZE * sizeof(struct stack_record *));
>>>>>>> +
>>>>>>> +       stack_table = vmalloc(size);
>>>>>>> +       memcpy(stack_table, stack_table_def, size);
>>>>>>
>>>>>> Looks like you are assuming stack_table_def already contains some data
>>>>>> by this point.
>>>>>> But if STACK_HASH_SIZE shrinks this memcpy() above will just copy some
>>>>>> part of the table, whereas the rest will be lost.
>>>>>> We'll need to:
>>>>>> - either explicitly decide we can afford losing this data (no idea how
>>>>>> bad this can potentially be),
>>>>>> - or disallow storing anything prior to full stackdepot initialization
>>>>>> (then we don't need stack_table_def),
>>>>>> - or carefully move all entries to the first part of the table.
>>>>>>
>>>>>> Alex
>>>>>>
>>>>>
>>>>> The hash for stack_table_def is computed using the run time parameter
>>>>> stack_hash_order, though stack_table_def is a bigger array it will only
>>>>> use the entries that are with in the run time configured STACK_HASH_SIZE
>>>>> range. so, there will be no data loss during copy.
>>>>
>>>> Do we expect any data to be stored into stack_table_def before
>>>> setup_stack_hash_order() is called?
>>>> If the answer is no, then we could probably drop stack_table_def and
>>>> allocate the table right in setup_stack_hash_order()?
>>>>
>>>
>>> Yes, we do see an allocation from stack depot even before init is called
>>> from kasan, thats the reason for having stack_table_def.
>>> This is the issue reported due to that on v2, so i added stack_table_def.
>>> https://lkml.org/lkml/2020/12/3/839
>>
>> But at that point STACK_HASH_SIZE is still equal to 1L <<
>> MAX_STACK_HASH_ORDER, isn't it?
>> Then we still need to take care of the records that fit into the
>> bigger array, but not the smaller one.
>>
> 
> At this point early_param is already called which sets stack_hash_order.
> So, STACK_HASH_SIZE will be set to this updated value and not
> MAX_STACK_HASH_SIZE.So, no additional entires in the bigger array.
> 
> Thanks,
> Vijay
> 

Let me know if there are any other concerns.

Thanks,
Vijay

>>> Thanks,
>>> Vijay
>>>
>>>>> Thanks,
>>>>> Vijay
>>>>>
>>>>> --
>>>>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
>>>>> member of Code Aurora Forum, hosted by The Linux Foundation
>>>>
>>>>
>>>>
>>>
>>> --
>>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
>>> member of Code Aurora Forum, hosted by The Linux Foundation
>>
>>
>>
> 

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of Code Aurora Forum, hosted by The Linux Foundation