On Sun, Aug 13, 2017 at 2:25 AM, Christoph Hellwig [off-list ref] wrote:

On Sat, Aug 12, 2017 at 07:44:14PM -0700, Dan Williams wrote:

quoted

How about MAP_SYNC == (MAP_SHARED|MAP_PRIVATE)? On older kernels that
should get -EINVAL, and on new kernels it means SYNC+SHARED.

Cute trick, but I'd hate to waster it just for our little flag.

How about:

#define __MAP_VALIDATE          MAP_SHARED|MAP_PRIVATE
#define MAP_SYNC                0x??? | __MAP_VALIDATE

so that we can reuse that trick for any new flag?

Yes, even better. I think that effectively kicks the need for mmap3()
down the road a bit.