linux-kernel - Re: [PATCH] null_blk: use sector_div instead of do

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Message-ID: <565CC47E.9020209@fb.com>
Date:	Mon, 30 Nov 2015 14:49:50 -0700
From:	Jens Axboe <axboe@...com>
To:	Arnd Bergmann <arnd@...db.de>,
	<linux-arm-kernel@...ts.infradead.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matias Bjorling <m@...rling.me>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] null_blk: use sector_div instead of do_div

On 11/27/2015 02:31 PM, Arnd Bergmann wrote:
> On Friday 27 November 2015 10:07:54 Linus Torvalds wrote:
>> On Fri, Nov 27, 2015 at 5:49 AM, Arnd Bergmann <arnd@...db.de> wrote:
>>> -       do_div(size, bs); /* convert size to pages */
>>> -       do_div(size, 256); /* concert size to pgs pr blk */
>>> +       sector_div(size, bs); /* convert size to pages */
>>> +       sector_div(size, 256); /* concert size to pgs pr blk */
>>
>> Ugh.
>>
>> Dividing by 256 should never be done with do_div() *or* sector-div.
>
> You are right, I missed what should have been an obvious simplification.
> FWIW, the division by 256 should now be optimized automatically when the
> new asm-generic do_div() implementation is used (which in turn caused
> the type mismatch warning that I'm trying to avoid). Of course that
> is no excuse for writing silly code like that, and it doesn't catch the
> cases where the argument is a power-of-two variable number.
>
> The first do_div() is also questionable: 'bs' is a global variable
> from a module parameter that defaults to 512 and is fixed to 4096
> when the device is used for lightnvm. I would guess that we run into
> bugs if this is ever set to a number that is not a power of two,
> smaller than 512, or larger than PAGE_SIZE.

We can use the shift, but honestly, it's just setup code. For hot paths, 
avoiding the div is definitely of higher priority. And for the sake of 
people copy/pasting or similar, making it a shift is probably a good idea.

-- 
Jens Axboe

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