Messages in this thread | | | Date | Mon, 14 Jan 2008 09:20:35 -0800 | From | Randy Dunlap <> | Subject | Re: [PATCH] Re-organization of PIDMAP_ENTRIES macro expansion |
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On Mon, 14 Jan 2008 16:20:35 +0530 Ratnadeep Joshi wrote:
> This patch tries to re-organize the macro expansion of PIDMAP_ENTRIES > (possibly) to a more clear one. > > Thanks, > - Ratnadeep Joshi > > diff --git a/include/linux/pid_namespace.h > b/include/linux/pid_namespace.h > index 1689e28..06e3e99 100644 > --- a/include/linux/pid_namespace.h > +++ b/include/linux/pid_namespace.h > @@ -12,7 +12,7 @@ struct pidmap { > void *page; > }; > > -#define PIDMAP_ENTRIES ((PID_MAX_LIMIT + 8*PAGE_SIZE - > 1)/PAGE_SIZE/8) > +#define PIDMAP_ENTRIES ((PID_MAX_LIMIT - 1)/PAGE_SIZE/8 + 1) > > struct pid_namespace { > struct kref kref;
I beg to disagree. There is a very common (idiomatic) way for doing this kind of calculation and the first/original expression is in that form, although it's a little difficult to recognize.
This common formula lives in linux/kernel.h:
#define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
where n is (AFAIK) numerator and d is divisor.
Example: One use of this is in linux/bitops.h:
#define BITS_TO_LONGS(nr) DIV_ROUND_UP(nr, BITS_PER_LONG)
This macro returns the number of longs that it takes to hold 'nr' bits. Expanded, it looks like:
nr_longs = (nr_bits + BITS_PER_LONG - 1) / BITS_PER_LONG;
The idiomatic formula handles boundary conditions very nicely.
Now for PIDMAP_ENTRIES, use 8 * PAGE_SIZE as the (d) part of DIV_ROUND_UP() and you can see that it is done correctly.
--- ~Randy
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