On Wed, Oct 28, 2015 at 02:44:28PM -0700, Eric Dumazet wrote:

Well, all this complexity goes away with a O_FD_FASTALLOC /
SOCK_FD_FASTALLOC bit in various fd allocations, which specifically
tells the kernel we do not care getting the lowest possible fd as POSIX
mandates.

... which won't do a damn thing for existing userland.

Except for legacy stuff and stdin/stdout/stderr games, I really doubt
lot of applications absolutely rely on the POSIX thing...

We obviously can't turn that into default behaviour, though.  BTW, what
distribution do you have in mind for those random descriptors?  Uniform
on [0,INT_MAX] is a bad idea for obvious reasons - you'll blow the
memory footprint pretty soon...