linux-kernel - Re: [PATCHv8] gpio: Remove VLA from gpiolib

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Message-ID: <CAHp75VciKgqYV_DB=h+zaVkk2+dmKJDhT02M4K8T1VGJA6KM0g@mail.gmail.com>
Date:   Fri, 18 May 2018 23:07:04 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        Lukas Wunner <lukas@...ner.de>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Phil Reid <preid@...ctromag.com.au>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCHv8] gpio: Remove VLA from gpiolib

On Fri, May 18, 2018 at 8:53 PM, Laura Abbott <labbott@...hat.com> wrote:
> The new challenge is to remove VLAs from the kernel
> (see https://lkml.org/lkml/2018/3/7/621) to eventually
> turn on -Wvla.
>
> Using a kmalloc array is the easy way to fix this but kmalloc is still
> more expensive than stack allocation. Introduce a fast path with a
> fixed size stack array to cover most chip with gpios below some fixed
> amount. The slow path dynamically allocates an array to cover those
> chips with a large number of gpios.

> +               unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)];
> +               unsigned long *mask, *bits;
>                 int first, j, ret;
>
> +               if (likely(chip->ngpio <= FASTPATH_NGPIO)) {
> +                       mask = fastpath;
> +               } else {
> +                       mask = kmalloc_array(2 * BITS_TO_LONGS(chip->ngpio),
> +                                          sizeof(*mask),
> +                                          can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> +                       if (!mask)
> +                               return -ENOMEM;
> +               }
> +
> +               bits = mask + BITS_TO_LONGS(chip->ngpio);
> +               memset(mask, 0, BITS_TO_LONGS(chip->ngpio) * sizeof(*mask));

Wouldn't be better

bitmap_zero(mask, chip->ngpio);

?

> +               bits = mask + BITS_TO_LONGS(chip->ngpio);
> +               memset(mask, 0, BITS_TO_LONGS(chip->ngpio) * sizeof(*mask));

Ditto.

-- 
With Best Regards,
Andy Shevchenko