linux-kernel - [PATCH v3 3/3] docs: riscv: Define behavior of mmap

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Message-Id: <20240130-use_mmap_hint_address-v3-3-8a655cfa8bcb@rivosinc.com>
Date: Tue, 30 Jan 2024 17:07:02 -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 v3 3/3] docs: riscv: Define behavior of mmap

Define mmap on riscv to not provide an address that uses more bits than
the hint address, if provided.

Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
---
 Documentation/arch/riscv/vm-layout.rst | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/Documentation/arch/riscv/vm-layout.rst b/Documentation/arch/riscv/vm-layout.rst
index 69ff6da1dbf8..e476b4386bd9 100644
--- a/Documentation/arch/riscv/vm-layout.rst
+++ b/Documentation/arch/riscv/vm-layout.rst
@@ -144,14 +144,8 @@ passing 0 into the hint address parameter of mmap. On CPUs with an address space
 smaller than sv48, the CPU maximum supported address space will be the default.
 
 Software can "opt-in" to receiving VAs from another VA space by providing
-a hint address to mmap. A hint address passed to mmap will cause the largest
-address space that fits entirely into the hint to be used, unless there is no
-space left in the address space. If there is no space available in the requested
-address space, an address in the next smallest available address space will be
-returned.
-
-For example, in order to obtain 48-bit VA space, a hint address greater than
-:code:`1 << 47` must be provided. Note that this is 47 due to sv48 userspace
-ending at :code:`1 << 47` and the addresses beyond this are reserved for the
-kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater
-than or equal to :code:`1 << 56` must be provided.
+a hint address to mmap. When a hint address is passed to mmap, the returned
+address will never use more bits than the hint address. For example, if a hint
+address of `1 << 40` is passed to mmap, a valid returned address will never use
+bits 41 through 63. If no mappable addresses are available in that range, mmap
+will return `MAP_FAILED`.

-- 
2.43.0