[PATCH RFC v2 1/4] mm: Add MAP_BELOW_HINT

Charlie Jenkins
2024-08-29T07:16:17Z

Some applications rely on placing data in free bits addresses allocated
by mmap. Various architectures (eg. x86, arm64, powerpc) restrict the
address returned by mmap to be less than the 48-bit address space,
unless the hint address uses more than 47 bits (the 48th bit is reserved
for the kernel address space).

To make this behavior explicit and more versatile across all
architectures, define a mmap flag that allows users to define an
arbitrary upper limit on addresses returned by mmap.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
---
include/uapi/asm-generic/mman-common.h | 1 +
tools/include/uapi/asm-generic/mman-common.h | 1 +
2 files changed, 2 insertions(+)

diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
index 6ce1f1ceb432..03ac13d9aa37 100644
--- a/include/uapi/asm-generic/mman-common.h
+++ b/include/uapi/asm-generic/mman-common.h
@@ -32,6 +32,7 @@

#define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be
* uninitialized */
+#define MAP_BELOW_HINT 0x8000000 /* give out address that is below (inclusive) hint address */

/*
* Flags for mlock
diff --git a/tools/include/uapi/asm-generic/mman-common.h b/tools/include/uapi/asm-generic/mman-common.h
index 6ce1f1ceb432..03ac13d9aa37 100644
--- a/tools/include/uapi/asm-generic/mman-common.h
+++ b/tools/include/uapi/asm-generic/mman-common.h
@@ -32,6 +32,7 @@

#define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be
* uninitialized */
+#define MAP_BELOW_HINT 0x8000000 /* give out address that is below (inclusive) hint address */

/*
* Flags for mlock

--
2.45.0