[ARM] Double check memmap is actually valid with a memmap has unexpected holes V2

pfn_valid() is meant to be able to tell if a given PFN has valid memmap
associated with it or not. In FLATMEM, it is expected that holes always
have valid memmap as long as there is valid PFNs either side of the hole.
In SPARSEMEM, it is assumed that a valid section has a memmap for the
entire section.

However, ARM and maybe other embedded architectures in the future free
memmap backing holes to save memory on the assumption the memmap is never
used. The page_zone linkages are then broken even though pfn_valid()
returns true. A walker of the full memmap must then do this additional
check to ensure the memmap they are looking at is sane by making sure the
zone and PFN linkages are still valid. This is expensive, but walkers of
the full memmap are extremely rare.

This was caught before for FLATMEM and hacked around but it hits again for
SPARSEMEM because the page_zone linkages can look ok where the PFN linkages
are totally screwed. This looks like a hatchet job but the reality is that
any clean solution would end up consumning all the memory saved by punching
these unexpected holes in the memmap. For example, we tried marking the
memmap within the section invalid but the section size exceeds the size of
the hole in most cases so pfn_valid() starts returning false where valid
memmap exists. Shrinking the size of the section would increase memory
consumption offsetting the gains.

This patch identifies when an architecture is punching unexpected holes
in the memmap that the memory model cannot automatically detect and sets
ARCH_HAS_HOLES_MEMORYMODEL. At the moment, this is restricted to EP93xx
which is the model sub-architecture this has been reported on but may expand
later. When set, walkers of the full memmap must call memmap_valid_within()
for each PFN and passing in what it expects the page and zone to be for
that PFN. If it finds the linkages to be broken, it assumes the memmap is
invalid for that PFN.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index e60ec54..9d02cdb 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -273,6 +273,7 @@ config ARCH_EP93XX
 	select HAVE_CLK
 	select COMMON_CLKDEV
 	select ARCH_REQUIRE_GPIOLIB
+	select ARCH_HAS_HOLES_MEMORYMODEL
 	help
 	  This enables support for the Cirrus EP93xx series of CPUs.
 
@@ -976,10 +977,9 @@ config OABI_COMPAT
 	  UNPREDICTABLE (in fact it can be predicted that it won't work
 	  at all). If in doubt say Y.
 
-config ARCH_FLATMEM_HAS_HOLES
+config ARCH_HAS_HOLES_MEMORYMODEL
 	bool
-	default y
-	depends on FLATMEM
+	default n
 
 # Discontigmem is deprecated
 config ARCH_DISCONTIGMEM_ENABLE