[PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

[Charlie Jenkins]

Create a personality flag ADDR_LIMIT_47BIT to support applications
that wish to transition from running in environments that support at
most 47-bit VAs to environments that support larger VAs. This
personality can be set to cause all allocations to be below the 47-bit
boundary. Using MAP_FIXED with mmap() will bypass this restriction.

Signed-off-by: Charlie Jenkins <charlie at rivosinc.com>
---
 include/uapi/linux/personality.h | 1 +
 mm/mmap.c                        | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/include/uapi/linux/personality.h b/include/uapi/linux/personality.h
index 49796b7756af..cd3b8c154d9b 100644
--- a/include/uapi/linux/personality.h
+++ b/include/uapi/linux/personality.h
@@ -22,6 +22,7 @@ enum {
 	WHOLE_SECONDS =		0x2000000,
 	STICKY_TIMEOUTS	=	0x4000000,
 	ADDR_LIMIT_3GB = 	0x8000000,
+	ADDR_LIMIT_47BIT = 	0x10000000,
 };
 
 /*
diff --git a/mm/mmap.c b/mm/mmap.c
index d0dfc85b209b..a5c7544853e5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1766,6 +1766,9 @@ unsigned long vm_unmapped_area(struct vm_unmapped_area_info *info)
 {
 	unsigned long addr;
 
+	if (current->personality & ADDR_LIMIT_47BIT)
+		info->high_limit = MIN(info->high_limit, BIT(47) - 1);
+
 	if (info->flags & VM_UNMAPPED_AREA_TOPDOWN)
 		addr = unmapped_area_topdown(info);
 	else

-- 
2.45.0