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Date: Tue, 27 Mar 2018 17:37:18 -0700
From: Laura Abbott <labbott@...hat.com>
To: Lukas Wunner <lukas@...ner.de>,
Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Linus Walleij <linus.walleij@...aro.org>,
Kees Cook <keescook@...omium.org>, linux-gpio@...r.kernel.org,
linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
Mathias Duckeck <m.duckeck@...bus.de>, Nandor Han <nandor.han@...com>,
Semi Malinen <semi.malinen@...com>, Patrice Chotard <patrice.chotard@...com>
Subject: Re: [PATCH 1/4] gpio: Remove VLA from gpiolib
On 03/18/2018 07:23 AM, Lukas Wunner wrote:
> On Sat, Mar 17, 2018 at 09:25:09AM +0100, Lukas Wunner wrote:
>> On Mon, Mar 12, 2018 at 04:00:36PM +0100, Rasmus Villemoes wrote:
>>> On 2018-03-10 01:10, Laura Abbott wrote:
>>>> @@ -2887,14 +2909,30 @@ void gpiod_set_array_value_complex(bool raw, bool can_sleep,
>>>>
>>>> while (i < array_size) {
>>>> struct gpio_chip *chip = desc_array[i]->gdev->chip;
>>>> - unsigned long mask[BITS_TO_LONGS(chip->ngpio)];
>>>> - unsigned long bits[BITS_TO_LONGS(chip->ngpio)];
>>>> + unsigned long *mask;
>>>> + unsigned long *bits;
>>>> int count = 0;
>>>>
>>>> + mask = kmalloc_array(BITS_TO_LONGS(chip->ngpio),
>>>> + sizeof(*mask),
>>>> + can_sleep ? GFP_KERNEL : GFP_ATOMIC);
>>>> +
>>>> + if (!mask)
>>>> + return;
>>>> +
>>>> + bits = kmalloc_array(BITS_TO_LONGS(chip->ngpio),
>>>> + sizeof(*bits),
>>>> + can_sleep ? GFP_KERNEL : GFP_ATOMIC);
>>>> +
>>>> + if (!bits) {
>>>> + kfree(mask);
>>>> + return;
>>>> + }
>>>> +
>>>> if (!can_sleep)
>>>> WARN_ON(chip->can_sleep);
>>>>
>>>> - memset(mask, 0, sizeof(mask));
>>>> + memset(mask, 0, sizeof(*mask));
>>>
>>> Other random thoughts: maybe two allocations for each loop iteration is
>>> a bit much. Maybe do a first pass over the array and collect the maximal
>>> chip->ngpio, do the memory allocation and freeing outside the loop (then
>>> you'd of course need to preserve the memset() with appropriate length
>>> computed). And maybe even just do one allocation, making bits point at
>>> the second half.
>>
>> I think those are great ideas because the function is kind of a hotpath
>> and usage of VLAs was motivated by the desire to make it fast.
>>
>> I'd go one step further and store the maximum ngpio of all registered
>> chips in a global variable (and update it in gpiochip_add_data_with_key()),
>> then allocate 2 * max_ngpio once before entering the loop (as you've
>> suggested). That would avoid the first pass to determine the maximum
>> chip->ngpio. In most systems max_ngpio will be < 64, so one or two
>> unsigned longs depending on the arch's bitness.
>
> Actually, scratch that. If ngpio is usually smallish, we can just
> allocate reasonably sized space for mask and bits on the stack,
> and fall back to the kcalloc slowpath only if chip->ngpio exceeds
> that limit. Basically the below (likewise compile-tested only),
> this is on top of Laura's patch, could be squashed together.
> Let me know what you think, thanks.
>
It seems like there's general consensus this is okay so I'm going
to fold it into the next version. If not, we can discuss again.
> -- >8 --
> Subject: [PATCH] gpio: Add fastpath to gpiod_get/set_array_value_complex()
>
> Signed-off-by: Lukas Wunner <lukas@...ner.de>
> ---
> drivers/gpio/gpiolib.c | 76 ++++++++++++++++++++++++--------------------------
> 1 file changed, 37 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index 429bc251392b..ffc67b0b866c 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -2432,6 +2432,8 @@ static int gpio_chip_get_multiple(struct gpio_chip *chip,
> return -EIO;
> }
>
> +#define FASTPATH_NGPIO 256
> +
> int gpiod_get_array_value_complex(bool raw, bool can_sleep,
> unsigned int array_size,
> struct gpio_desc **desc_array,
> @@ -2441,27 +2443,24 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
>
> while (i < array_size) {
> struct gpio_chip *chip = desc_array[i]->gdev->chip;
> - unsigned long *mask;
> - unsigned long *bits;
> + unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)];
> + unsigned long *slowpath = NULL, *mask, *bits;
> int first, j, ret;
>
> - mask = kcalloc(BITS_TO_LONGS(chip->ngpio),
> - sizeof(*mask),
> - can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> -
> - if (!mask)
> - return -ENOMEM;
> -
> - bits = kcalloc(BITS_TO_LONGS(chip->ngpio),
> - sizeof(*bits),
> - can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> -
> - if (!bits) {
> - kfree(mask);
> - return -ENOMEM;
> + if (likely(chip->ngpio <= FASTPATH_NGPIO)) {
> + memset(fastpath, 0, sizeof(fastpath));
> + mask = fastpath;
> + bits = fastpath + BITS_TO_LONGS(FASTPATH_NGPIO);
> + } else {
> + slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
> + sizeof(*slowpath),
> + can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> + if (!slowpath)
> + return -ENOMEM;
> + mask = slowpath;
> + bits = slowpath + BITS_TO_LONGS(chip->ngpio);
> }
>
> -
> if (!can_sleep)
> WARN_ON(chip->can_sleep);
>
> @@ -2478,8 +2477,8 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
>
> ret = gpio_chip_get_multiple(chip, mask, bits);
> if (ret) {
> - kfree(bits);
> - kfree(mask);
> + if (slowpath)
> + kfree(slowpath);
> return ret;
> }
>
> @@ -2493,8 +2492,9 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
> value_array[j] = value;
> trace_gpio_value(desc_to_gpio(desc), 1, value);
> }
> - kfree(bits);
> - kfree(mask);
> +
> + if (slowpath)
> + kfree(slowpath);
> }
> return 0;
> }
> @@ -2699,24 +2699,22 @@ int gpiod_set_array_value_complex(bool raw, bool can_sleep,
>
> while (i < array_size) {
> struct gpio_chip *chip = desc_array[i]->gdev->chip;
> - unsigned long *mask;
> - unsigned long *bits;
> + unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)];
> + unsigned long *slowpath = NULL, *mask, *bits;
> int count = 0;
>
> - mask = kcalloc(BITS_TO_LONGS(chip->ngpio),
> - sizeof(*mask),
> - can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> -
> - if (!mask)
> - return -ENOMEM;
> -
> - bits = kcalloc(BITS_TO_LONGS(chip->ngpio),
> - sizeof(*bits),
> - can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> -
> - if (!bits) {
> - kfree(mask);
> - return -ENOMEM;
> + if (likely(chip->ngpio <= FASTPATH_NGPIO)) {
> + memset(fastpath, 0, sizeof(fastpath));
> + mask = fastpath;
> + bits = fastpath + BITS_TO_LONGS(FASTPATH_NGPIO);
> + } else {
> + slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
> + sizeof(*slowpath),
> + can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> + if (!slowpath)
> + return -ENOMEM;
> + mask = slowpath;
> + bits = slowpath + BITS_TO_LONGS(chip->ngpio);
> }
>
> if (!can_sleep)
> @@ -2753,8 +2751,8 @@ int gpiod_set_array_value_complex(bool raw, bool can_sleep,
> if (count != 0)
> gpio_chip_set_multiple(chip, mask, bits);
>
> - kfree(mask);
> - kfree(bits);
> + if (slowpath)
> + kfree(slowpath);
> }
> return 0;
> }
>
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