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

[Vijayanand Jitta]

On 12/14/2020 3:04 PM, Alexander Potapenko wrote:
> On Mon, Dec 14, 2020 at 5:02 AM Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 12/11/2020 6:55 PM, Alexander Potapenko wrote:
>>> On Fri, Dec 11, 2020 at 1:45 PM Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>> On 12/11/2020 2:06 PM, Alexander Potapenko wrote:
>>>>> On Thu, Dec 10, 2020 at 6:01 AM <vjitta@xxxxxxxxxxxxxx> wrote:
>>>>>>
>>>>>> From: Yogesh Lal <ylal@xxxxxxxxxxxxxx>
>>>>>>
>>>>>> 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

>> 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