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Date: Sun, 18 Mar 2018 21:34:12 +0100
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: Lukas Wunner <lukas@...ner.de>,
Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Laura Abbott <labbott@...hat.com>,
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 2018-03-18 15:23, Lukas Wunner wrote:
>>>
>>> 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,
Yes.
> and fall back to the kcalloc slowpath only if chip->ngpio exceeds
> that limit.
Well, I'd suggest not adding that fallback code now, but simply add a
check in gpiochip_add_data_with_key to ensure ngpio is sane (and refuse
to register the chip otherwise), at least if we know that every
currently supported/known chip is covered by the 256 (?). That keeps the
code simple and fast, and then if somebody has a chip with 40000 gpio
lines, we can add a fallback path. Or we could consider alternative
solutions, to avoid a 10000 byte GFP_ATOMIC allocation (maybe hang a
pre-allocation off the gpio_chip; that's only two more bits per
descriptor, and there's already a whole gpio_desc for each - but not
sure about the locking in that case).
Rasmus
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