Re: [PATCH] bitmap: Fix optimization of bitmap_set/clear for big-endian machines

[Paul Mackerras]

On Wed, Oct 25, 2017 at 07:39:48AM +0000, Matthew Wilcox wrote:
> Hang on, don't tell me you found this by inspection.  Are you not running the bitmap testcase, enabled by CONFIG_TEST_BITMAP?  Either that should be producing an error, or there's a missing test case, or your inspection is wrong ...

I did find it by inspection.  I was looking for a version of the
bitmap_* API that does little-endian style bitmaps on all systems, and
the inline bitmap_set() does that in the case where it calls memset,
but not in the case where it calls __bitmap_set.

I'll fire up a big-endian system tomorrow when I get to work to run
the test case.  (PPC64 is almost entirely little-endian these days as
far as the IBM POWER systems are concerned.)

In any case, it's pretty clearly wrong as it is.  On a big-endian
64-bit system, bitmap_set(p, 56, 16) should set bytes 0 and 15 to
0xff, and there's no way a single memset can do that.

Paul.

(and yes, I stuffed up the address for lkml)