code review 4634086: runtime: don't use twice the memory with grsec-like kernels

runtime: don't use twice the memory with grsec-like kernels

grsec needs the FIXED flag to be provided to mmap, which
works now.  That said, when the allocation fails to be made
in the specific address, we're still given back a writable
page.  This change will unmap that page to avoid using
twice the amount of memory needed.

It'd also be pretty easy to avoid the extra system calls
once we detected that the flag is needed, but I'm not sure
if that edge case is worth the effort.

[niemeyer, 2011-06-23 20:43:35]

Hello golang-dev@googlegroups.com (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://go.googlecode.com/hg/

[rsc, 2011-06-23 21:34:54]

LGTM

but we're not really using the memory anyway.

[niemeyer, 2011-06-24 03:30:09]

*** Submitted as http://code.google.com/p/go/source/detail?r=5b8d744d0d63 ***

runtime: don't use twice the memory with grsec-like kernels

grsec needs the FIXED flag to be provided to mmap, which
works now.  That said, when the allocation fails to be made
in the specific address, we're still given back a writable
page.  This change will unmap that page to avoid using
twice the amount of memory needed.

It'd also be pretty easy to avoid the extra system calls
once we detected that the flag is needed, but I'm not sure
if that edge case is worth the effort.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4634086