linux-kernel - Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH

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Message-ID: <CAG_fn=VOHag5AUwFbOj_cV+7RDAk8UnjjqEtv2xmkSDb_iTYcQ@mail.gmail.com>
Date:   Mon, 14 Dec 2020 10:34:41 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Vijayanand Jitta <vjitta@...eaurora.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        dan.j.williams@...el.com, broonie@...nel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>, qcai@...hat.com,
        ylal@...eaurora.org, vinmenon@...eaurora.org
Subject: Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH_SIZE

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

> 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



-- 
Alexander Potapenko
Software Engineer

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