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

[Alexander Potapenko <glider@google.com>]

On Wed, Dec 16, 2020 at 4:43 AM Vijayanand Jitta <vjitta@codeaurora.org> 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@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.

I still think there are implicit assumptions that should at least be
written down in 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.

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.

> 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



-- 
Alexander Potapenko
Software Engineer

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