[PATCH v3 0/3] RISC-V: mm: do not treat hint addr on mmap as the upper bound to search

[Yangyu Chen]

Previous patch series[1][2] changes a mmap behavior that treats the hint
address as the upper bound of the mmap address range. The motivation of the
previous patch series is that some user space software may assume 48-bit
address space and use higher bits to encode some information, which may
collide with large virtual address space mmap may return. However, to make
sv48 by default, we don't need to change the meaning of the hint address on
mmap as the upper bound of the mmap address range. This behavior breaks
some user space software like Chromium that gets ENOMEM error when the hint
address + size is not big enough, as specified in [3].

Other ISAs with larger than 48-bit virtual address space like x86, arm64,
and powerpc do not have this special mmap behavior on hint address. They
all just make 48-bit / 47-bit virtual address space by default, and if a
user space software wants to large virtual address space, it only need to
specify a hint address larger than 48-bit / 47-bit.

Thus, this patch series change mmap to use sv48 by default but does not
treat the hint address as the upper bound of the mmap address range. After
this patch, the behavior of mmap will align with existing behavior on other
ISAs with larger than 48-bit virtual address space like x86, arm64, and
powerpc. The user space software will no longer need to rewrite their code
to fit with this special mmap behavior only on RISC-V.

Note: Charlie also created another series [4] to completely remove the
arch_get_mmap_end and arch_get_mmap_base behavior based on the hint address
and size. However, this will cause programs like Go and Java, which need to
store information in the higher bits of the pointer, to fail on Sv57
machines.

Changes in v3:
- Rebase to newest master
- Changes some information in cover letter after patchset [2]
- Use patch [5] to patch selftests
- Link to v2: https://lore.kernel.org/linux-riscv/tencent_B2D0435BC011135736262764B511994F4805@qq.com/

Changes in v2:
- correct arch_get_mmap_end and arch_get_mmap_base
- Add description in documentation about mmap behavior on kernel v6.6-6.7.
- Improve commit message and cover letter
- Rebase to newest riscv/for-next branch
- Link to v1: https://lore.kernel.org/linux-riscv/tencent_F3B3B5AB1C9D704763CA423E1A41F8BE0509@qq.com/

[1] https://lore.kernel.org/linux-riscv/20230809232218.849726-1-charlie@rivosinc.com/
[2] https://lore.kernel.org/linux-riscv/20240130-use_mmap_hint_address-v3-0-8a655cfa8bcb@rivosinc.com/
[3] https://lore.kernel.org/linux-riscv/MEYP282MB2312A08FF95D44014AB78411C68D2@MEYP282MB2312.AUSP282.PROD.OUTLOOK.COM/
[4] https://lore.kernel.org/linux-riscv/20240826-riscv_mmap-v1-0-cd8962afe47f@rivosinc.com/
[5] https://lore.kernel.org/linux-riscv/20240826-riscv_mmap-v1-2-cd8962afe47f@rivosinc.com/

Charlie Jenkins (1):
  riscv: selftests: Remove mmap hint address checks

Yangyu Chen (2):
  RISC-V: mm: not use hint addr as upper bound
  Documentation: riscv: correct sv57 kernel behavior

 Documentation/arch/riscv/vm-layout.rst        | 43 ++++++++----
 arch/riscv/include/asm/processor.h            | 20 ++----
 .../selftests/riscv/mm/mmap_bottomup.c        |  2 -
 .../testing/selftests/riscv/mm/mmap_default.c |  2 -
 tools/testing/selftests/riscv/mm/mmap_test.h  | 67 -------------------
 5 files changed, 36 insertions(+), 98 deletions(-)

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2.45.2