Charlie Jenkins <charlie@rivosinc.com> says:
On riscv, mmap currently returns an address from the largest address
space that can fit entirely inside of the hint address. This makes it
such that the hint address is almost never returned. This patch raises
the mappable area up to and including the hint address. This allows mmap
to often return the hint address, which allows a performance improvement
over searching for a valid address as well as making the behavior more
similar to other architectures.
Note that a previous patch introduced stronger semantics compared to
other architectures for riscv mmap. On riscv, mmap will not use bits in
the upper bits of the virtual address depending on the hint address. On
other architectures, a random address is returned in the address space
requested. On all architectures the hint address will be returned if it
is available. This allows riscv applications to configure how many bits
in the virtual address should be left empty. This has the two benefits
of being able to request address spaces that are smaller than the
default and doesn't require the application to know the page table
layout of riscv.
* b4-shazam-merge:
docs: riscv: Define behavior of mmap
selftests: riscv: Generalize mm selftests
riscv: mm: Use hint address in mmap if available
Link: https://lore.kernel.org/r/20240130-use_mmap_hint_address-v3-0-8a655cfa8bcb@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Add tests that enforce mmap hint address behavior. mmap should default
to sv48. mmap will provide an address at the highest address space that
can fit into the hint address, unless the hint address is less than sv39
and not 0, then it will return a sv39 address.
These tests are split into two files: mmap_default.c and mmap_bottomup.c
because a new process must be exec'd in order to change the mmap layout.
The run_mmap.sh script sets the stack to be unlimited for the
mmap_bottomup.c test which triggers a bottomup layout.
Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20230809232218.849726-3-charlie@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
