iter and normal ops on /dev/zero & co, was Re: remove the last set_fs() in common code, and remove it for x86 and powerpc

[Christoph Hellwig]

On Wed, Aug 19, 2020 at 09:16:59AM +0200, Christophe Leroy wrote:
> I made a test with only the first patch of your series: That's definitely 
> the culprit. With only that patch applies, the duration is 6.64 seconds, 
> that's a 25% degradation.

For the record: the first patch is:

     mem: remove duplicate ops for /dev/zero and /dev/null

So these micro-optimizations matter at least for some popular
benchmarks.  It would be easy to drop, but that means we either:

 - can't support kernel_read/write on these files, which should not
   matter

or
 
 - have to drop the check for both ops being present

Al, what do you think?