mm,kfence: decouple kfence from page granularity mapping judgement

JIRA: https://issues.redhat.com/browse/RHEL-2769

Backport a standalone change to decouple KFENCE from the early mapping
granularity decision that spans over all the available RAM.

Signed-off-by: Eric Chanudet echanude@redhat.com

How to test

A regular kernel (as opposed to -debug kernel) has CONFIG_KFENCE=y set. Without this series, it causes all the linear mappings to be made at page granularity independently of rodata= setting. This uses more memory to hold the numerous, smaller, mappings in page-tables.

# sed -n 's/.*\(rodata=[^ ]\+\).*/\1/p' /proc/cmdline
# sed -n '/MemTotal:/p' /proc/meminfo
MemTotal:       35820768 kB

• Without this series, adding rodata=off to the kernel parameters has no effect on the linear mappings and /proc/meminfo reports the same MemTotal with or without rodata=off.

# sed -n 's/.*\(rodata=[^ ]\+\).*/\1/p' /proc/cmdline
rodata=off
# sed -n '/MemTotal:/p' /proc/meminfo
MemTotal:       35820768 kB

• With this series, adding rodata=off to the kernel parameters will have /proc/meminfo report additional memory available in MemTotal.

# sed -n 's/.*\(rodata=[^ ]\+\).*/\1/p' /proc/cmdline
rodata=off
# sed -n '/MemTotal:/p' /proc/meminfo 
MemTotal:       35892480 kB