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

[Vijayanand Jitta]

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

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
> 
> 
> 

-- 
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member of Code Aurora Forum, hosted by The Linux Foundation