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

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Date:   Fri, 11 Dec 2020 09:36:04 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     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 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.

> -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.

> +static struct stack_record *stack_table_def[MAX_STACK_HASH_SIZE] __initdata = {
> +       [0 ...  MAX_STACK_HASH_SIZE - 1] = NULL
>  };
> +static struct stack_record **stack_table __refdata = stack_table_def;
> +
> +static int __init setup_stack_hash_order(char *str)
> +{
> +       kstrtouint(str, 0, &stack_hash_order);
> +       if (stack_hash_order > MAX_STACK_HASH_ORDER)
> +               stack_hash_order = MAX_STACK_HASH_ORDER;
> +       return 0;
> +}
> +early_param("stack_hash_order", setup_stack_hash_order);
> +
> +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