[RFC] x86/mm: only allow memmap=XX!YY over existing RAM

[Yigal Korman <yigal-/8YdC2HfS5554TAoqtyWWQ@public.gmane.org>]

Before this patch, passing a range that is beyond the physical memory
range will succeed, the user will see a /dev/pmem0 and will be able to
access it. Reads will always return 0 and writes will be silently
ignored.

I've gotten more than one bug report about mkfs.{xfs,ext4} or nvml
failing that were eventually tracked down to be wrong values passed to
memmap.

This patch prevents the above issue by instead of adding a new memory
range, only update a RAM memory range with the PRAM type. This way,
passing the wrong memmap will either not give you a pmem at all or give
you a smaller one that actually has RAM behind it.

And if someone still needs to fake a pmem that doesn't have RAM behind
it, they can simply do memmap=XX@YY,XX!YY.

Signed-off-by: Yigal Korman <yigal-/8YdC2HfS5554TAoqtyWWQ@public.gmane.org>
---
 arch/x86/kernel/e820.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 569c1e4..bcd2ebb1 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -877,7 +877,7 @@ static int __init parse_memmap_one(char *p)
 		e820_add_region(start_at, mem_size, E820_RESERVED);
 	} else if (*p == '!') {
 		start_at = memparse(p+1, &p);
-		e820_add_region(start_at, mem_size, E820_PRAM);
+		e820_update_range(start_at, mem_size, E820_RAM, E820_PRAM);
 	} else
 		e820_remove_range(mem_size, ULLONG_MAX - mem_size, E820_RAM, 1);
 
-- 
1.9.3