RFC mm: PG_reserved cleanups and documentation

I was recently going over all users of PG_reserved. Short story: it is difficult and sometimes not really clear if setting/checking for PG_reserved is only a relict from the past. Easy to break things.

I had way more cleanups in this series inititally,
but some architectures take PG_reserved as a way to apply a different caching strategy (for MMIO pages). So I decided to only include the most obvious changes (that are less likely to break something).

So let's see if the documentation update for PG_reserved I crafted actually covers most cases or if there is plenty more.

Most notably, for device memory we can hopefully soon stop setting it PG_reserved

I only briefly tested this on s390x.

David Hildenbrand (7):
agp: efficeon: no need to set PG_reserved on GATT tables s390/vdso: don't clear PG_reserved powerpc/vdso: don't clear PG_reserved riscv/vdso: don't clear PG_reserved
m68k/mm: use __ClearPageReserved()
arm64: kexec: no need to ClearPageReserved()
mm: better document PG_reserved

 arch/arm64/kernel/machine_kexec.c |  1 -
 arch/m68k/mm/memory.c             |  2 +-
 arch/powerpc/kernel/vdso.c        |  2 --
 arch/riscv/kernel/vdso.c          |  1 -
 arch/s390/kernel/vdso.c           |  2 --
 drivers/char/agp/efficeon-agp.c   |  2 --
 include/linux/page-flags.h        | 18 ++++++++++++++++--
 7 files changed, 17 insertions(+), 11 deletions(-)

2.17.2