[patch 0/2] x86_64: preserve large page text mappings with CONFIG_DEBUG_RODATA

[Suresh Siddha <suresh.b.siddha@intel.com>]

Most of the distributions enable CONFIG_DEBUG_RODATA by default, with the
assumption that the debug protection checks come with no cost. Our OLTP tests
showed 1% improvement (with 2% reduction in system time and 1% increase in
iowait idle time) by disabling CONFIG_DEBUG_RODATA on a two socket
NHM-EP system.

CONFIG_DEBUG_RODATA breaks some of the kernel text/data large page
mappings to small pages, thus causing this regression. These two
patches address this issue.

Experiments with EL5 kernel and mainline show that the first patch in the series
(preserve large page mapping for 1st 2MB kernel txt with CONFIG_DEBUG_RODATA)
should recover most of the above mentioned regression. We are doing
couple of more OLTP tests for the confirmation.

thanks,
suresh