Messages in this thread | | | Subject | Re: [PATCH] null_blk: use sector_div instead of do_div | From | Jens Axboe <> | Date | Mon, 30 Nov 2015 14:49:50 -0700 |
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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@arndb.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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