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

[Vijayanand Jitta]

On 12/16/2020 6:41 PM, Alexander Potapenko wrote:
> On Wed, Dec 16, 2020 at 2:06 PM Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 12/16/2020 1:56 PM, Alexander Potapenko wrote:
>>> On Wed, Dec 16, 2020 at 4:43 AM Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>> 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@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
>>>>>
>>>>
>>>> Let me know if there are any other concerns.
>>>
>>> I still think there are implicit assumptions that should at least be
>>> written down in the comments.
>>
>> Sure, will add the comments.
>>
>>> As far as I understand the code, here is what happens here:
>>>
>>> 0. No stacks are recorded.
>>> 1. early_param is called to set stack_hash_order to a value
>>> potentially smaller than MAX_STACK_HASH_SIZE.
>>> 2. KASAN (or other users) records some stacks into stack_table_def
>>> (capped at new STACK_HASH_SIZE)
>>> 3. init_stackdepot() allocates a new stack_table and copies the
>>> contents of stack_table_def into it
>>> 4. Further stacks are recorded into the new stack_table
>>>
>>> If this is how the things work, I agree we don't need to account for
>>> the part of stack_table_def past STACK_HASH_SIZE.
>>> Not allocating stack_table when setting stack_hash_order is probably
>>> also justified, as we don't have SLAB or vmalloc at that point.
>>>
>>
>> That's Right.
>>
>>> But I am still curious if a runtime parameter that disables the
>>> stackdepot completely will solve your problem.
>>> Allocating a small amount of memory when you actually don't want to
>>> allocate any sounds suboptimal.
>>>
>>
>> I think disabling stack depot completely should be fine, we can make
>> STACK_HASH_SIZE as runtime parameter instead of stack_hash_order and set
>> stack_hash_size to 0 to disable stack depot completely.
> 
> To reiterate, I think you don't need a tunable stack_hash_order
> parameter if the only use case is to disable the stack depot.
> Maybe it is enough to just add a boolean flag?

There are multiple users of stackdepot they might still want to use
stack depot but with a lower memory footprint instead of MAX_SIZE
so, a configurable size might help here ?

> Or even go further and disable the stack depot in the same place that
> disables page owner, as the user probably doesn't want to set two
> flags instead of one?
> 

Since, page owner is not the only user of stack depot we can't take that
decision of disabling stack depot if page owner is disabled.

>> Minchan,
>> This should be fine right ? Do you see any issue with disabling
>> stack depot completely ?
>>
>> Thanks,
>> Vijay
>>
>>>> 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
>>>
>>>
>>>
>>
>> --
>> 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