From 5a78977262f47dd52a4880689a2ca3d21dbf149a Mon Sep 17 00:00:00 2001
From: Alice Ryhl <aliceryhl@google.com>
Date: Fri, 16 May 2025 19:32:19 +0000
Subject: mm: rust: make CONFIG_MMU ifdefs more narrow

Currently the entire kernel::mm module is ifdef'd out when CONFIG_MMU=n.
However, there are some downstream users of the module in
rust/kernel/task.rs and rust/kernel/miscdevice.rs. Thus, update the cfgs
so that only MmWithUserAsync is removed with CONFIG_MMU=n.

The code is moved into a new file, since the #[cfg()] annotation
otherwise has to be duplicated several times.

Link: https://lkml.kernel.org/r/20250516193219.2987032-1-aliceryhl@google.com
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505071753.kldNHYVQ-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202505072116.eSYC8igT-lkp@intel.com/
Fixes: 5bb9ed6cdfeb ("mm: rust: add abstraction for struct mm_struct")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 rust/kernel/mm/mmput_async.rs | 68 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 rust/kernel/mm/mmput_async.rs

(limited to 'rust/kernel/mm/mmput_async.rs')

diff --git a/rust/kernel/mm/mmput_async.rs b/rust/kernel/mm/mmput_async.rs
new file mode 100644
index 000000000000..9289e05f7a67
--- /dev/null
+++ b/rust/kernel/mm/mmput_async.rs
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Version of `MmWithUser` using `mmput_async`.
+//!
+//! This is a separate file from `mm.rs` due to the dependency on `CONFIG_MMU=y`.
+#![cfg(CONFIG_MMU)]
+
+use crate::{
+    bindings,
+    mm::MmWithUser,
+    types::{ARef, AlwaysRefCounted},
+};
+use core::{ops::Deref, ptr::NonNull};
+
+/// A wrapper for the kernel's `struct mm_struct`.
+///
+/// This type is identical to `MmWithUser` except that it uses `mmput_async` when dropping a
+/// refcount. This means that the destructor of `ARef<MmWithUserAsync>` is safe to call in atomic
+/// context.
+///
+/// # Invariants
+///
+/// Values of this type are always refcounted using `mmget`. The value of `mm_users` is non-zero.
+#[repr(transparent)]
+pub struct MmWithUserAsync {
+    mm: MmWithUser,
+}
+
+// SAFETY: It is safe to call `mmput_async` on another thread than where `mmget` was called.
+unsafe impl Send for MmWithUserAsync {}
+// SAFETY: All methods on `MmWithUserAsync` can be called in parallel from several threads.
+unsafe impl Sync for MmWithUserAsync {}
+
+// SAFETY: By the type invariants, this type is always refcounted.
+unsafe impl AlwaysRefCounted for MmWithUserAsync {
+    #[inline]
+    fn inc_ref(&self) {
+        // SAFETY: The pointer is valid since self is a reference.
+        unsafe { bindings::mmget(self.as_raw()) };
+    }
+
+    #[inline]
+    unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: The caller is giving up their refcount.
+        unsafe { bindings::mmput_async(obj.cast().as_ptr()) };
+    }
+}
+
+// Make all `MmWithUser` methods available on `MmWithUserAsync`.
+impl Deref for MmWithUserAsync {
+    type Target = MmWithUser;
+
+    #[inline]
+    fn deref(&self) -> &MmWithUser {
+        &self.mm
+    }
+}
+
+impl MmWithUser {
+    /// Use `mmput_async` when dropping this refcount.
+    #[inline]
+    pub fn into_mmput_async(me: ARef<MmWithUser>) -> ARef<MmWithUserAsync> {
+        // SAFETY: The layouts and invariants are compatible.
+        unsafe { ARef::from_raw(ARef::into_raw(me).cast()) }
+    }
+}
-- 
cgit