From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Lukas Wunner <lukas@wunner.de>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Laura Abbott <labbott@redhat.com>,
Linus Walleij <linus.walleij@linaro.org>,
Kees Cook <keescook@chromium.org>,
linux-gpio@vger.kernel.org, linux-kernel@vger.kernel.org,
kernel-hardening@lists.openwall.com,
Mathias Duckeck <m.duckeck@kunbus.de>,
Nandor Han <nandor.han@ge.com>,
Semi Malinen <semi.malinen@ge.com>,
Patrice Chotard <patrice.chotard@st.com>
Subject: Re: [PATCH 1/4] gpio: Remove VLA from gpiolib
Date: Sun, 18 Mar 2018 21:34:12 +0100 [thread overview]
Message-ID: <0f17eb05-c183-bec9-0076-5ddd00d70f15@rasmusvillemoes.dk> (raw)
In-Reply-To: <20180318142327.GA23761@wunner.de>
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
next prev parent reply other threads:[~2018-03-18 20:34 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-10 0:10 [PATCH 0/4] VLA removal from the GPIO subsystem Laura Abbott
2018-03-10 0:10 ` [PATCH 1/4] gpio: Remove VLA from gpiolib Laura Abbott
2018-03-12 15:00 ` Rasmus Villemoes
2018-03-12 23:40 ` Laura Abbott
2018-03-13 7:23 ` Rasmus Villemoes
2018-03-17 8:25 ` Lukas Wunner
2018-03-18 14:23 ` Lukas Wunner
2018-03-18 20:34 ` Rasmus Villemoes [this message]
2018-03-19 7:00 ` Lukas Wunner
2018-03-19 15:09 ` Andy Shevchenko
2018-03-28 0:37 ` Laura Abbott
2018-03-28 3:54 ` Lukas Wunner
2018-03-10 0:10 ` [PATCH 2/4] gpio: Remove VLA from MAX3191X driver Laura Abbott
2018-03-26 9:07 ` Linus Walleij
2018-03-10 0:10 ` [PATCH 3/4] gpio: Remove VLA from xra1403 driver Laura Abbott
2018-03-12 6:06 ` EXT: " Nandor Han
2018-03-26 9:09 ` Linus Walleij
2018-03-28 7:27 ` Geert Uytterhoeven
2018-03-28 17:27 ` Laura Abbott
2018-04-04 12:53 ` Linus Walleij
2018-03-10 0:10 ` [PATCH 4/4] gpio: Remove VLA from stmpe driver Laura Abbott
2018-03-13 9:13 ` Phil Reid
2018-03-14 0:18 ` Laura Abbott
2018-03-14 1:16 ` Laura Abbott
2018-03-14 2:55 ` Phil Reid
2018-03-13 9:42 ` [PATCH 0/4] VLA removal from the GPIO subsystem Linus Walleij
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