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: 5bb9ed6cdf ("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.rs

@@ -10,7 +10,6 @@
 //! control what happens when userspace reads or writes to that region of memory.
 //!
 //! C header: [`include/linux/mm.h`](srctree/include/linux/mm.h)
-#![cfg(CONFIG_MMU)]
 
 use crate::{
     bindings,
@@ -21,6 +20,10 @@
 pub mod virt;
 use virt::VmaRef;
 
+#[cfg(CONFIG_MMU)]
+pub use mmput_async::MmWithUserAsync;
+mod mmput_async;
+
 /// A wrapper for the kernel's `struct mm_struct`.
 ///
 /// This represents the address space of a userspace process, so each process has one `Mm`
@@ -111,50 +114,6 @@ fn deref(&self) -> &Mm {
     }
 }
 
-/// 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
-    }
-}
-
 // These methods are safe to call even if `mm_users` is zero.
 impl Mm {
     /// Returns a raw pointer to the inner `mm_struct`.
@@ -206,13 +165,6 @@ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a MmWithUser {
         unsafe { &*ptr.cast() }
     }
 
-    /// 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()) }
-    }
-
     /// Attempt to access a vma using the vma read lock.
     ///
     /// This is an optimistic trylock operation, so it may fail if there is contention. In that

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()) }
+    }
+}