On 11/27/2015 02:31 PM, Arnd Bergmann wrote:

On Friday 27 November 2015 10:07:54 Linus Torvalds wrote:

quoted

On Fri, Nov 27, 2015 at 5:49 AM, Arnd Bergmann [off-list ref] wrote:

quoted

-       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