linux-kernel - [PATCH v2 0/3] riscv: mm: Use hint address in mmap if available

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Message-Id: <20240130-use_mmap_hint_address-v2-0-f34ebfd33053@rivosinc.com>
Date: Tue, 30 Jan 2024 11:04:29 -0800
From: Charlie Jenkins <charlie@...osinc.com>
To: Alexandre Ghiti <alexghiti@...osinc.com>, 
 Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Shuah Khan <shuah@...nel.org>, Jonathan Corbet <corbet@....net>, 
 Yangyu Chen <cyy@...self.name>
Cc: linux-mm@...ck.org, linux-riscv@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 linux-doc@...r.kernel.org, Charlie Jenkins <charlie@...osinc.com>
Subject: [PATCH v2 0/3] riscv: mm: Use hint address in mmap if available

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.

Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
---
Changes in v2:
- Add back forgotten "mmap_end = STACK_TOP_MAX"
- Link to v1: https://lore.kernel.org/r/20240129-use_mmap_hint_address-v1-0-4c74da813ba1@rivosinc.com

---
Charlie Jenkins (3):
      riscv: mm: Use hint address in mmap if available
      selftests: riscv: Generalize mm selftests
      docs: riscv: Define behavior of mmap

 Documentation/arch/riscv/vm-layout.rst           | 16 ++--
 arch/riscv/include/asm/processor.h               | 22 +++---
 tools/testing/selftests/riscv/mm/mmap_bottomup.c | 20 +----
 tools/testing/selftests/riscv/mm/mmap_default.c  | 20 +----
 tools/testing/selftests/riscv/mm/mmap_test.h     | 93 +++++++++++++-----------
 5 files changed, 67 insertions(+), 104 deletions(-)
---
base-commit: 556e2d17cae620d549c5474b1ece053430cd50bc
change-id: 20240119-use_mmap_hint_address-f9f4b1b6f5f1
-- 
- Charlie