[PATCH v15 1/9] mm: rust: add abstraction for struct mm_struct

Alice Ryhl
2025-03-04T11:51:42Z

These abstractions allow you to reference a `struct mm_struct` using
both mmgrab and mmget refcounts. This is done using two Rust types:

* Mm - represents an mm_struct where you don't know anything about the
value of mm_users.
* MmWithUser - represents an mm_struct where you know at compile time
that mm_users is non-zero.

This allows us to encode in the type system whether a method requires
that mm_users is non-zero or not. For instance, you can always call
`mmget_not_zero` but you can only call `mmap_read_lock` when mm_users is
non-zero.

The struct is called Mm to keep consistency with the C side.

The ability to obtain `current->mm` is added later in this series.

Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/helpers/helpers.c | 1 +
rust/helpers/mm.c | 39 +++++++++
rust/kernel/lib.rs | 1 +
rust/kernel/mm.rs | 209 +++++++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 250 insertions(+)

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 0640b7e115be1553549312dcfdf842bcae3bde1b..97cfc2d29f5ee0441a5938eb6b39c51dfa337cb4 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -18,6 +18,7 @@
#include "io.c"
#include "jump_label.c"
#include "kunit.c"
+#include "mm.c"
#include "mutex.c"
#include "page.c"
#include "platform.c"
diff --git a/rust/helpers/mm.c b/rust/helpers/mm.c
new file mode 100644
index 0000000000000000000000000000000000000000..7201747a5d314b2b120b30c0b906715c04ca77a5
--- /dev/null
+++ b/rust/helpers/mm.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/mm.h>
+#include <linux/sched/mm.h>
+
+void rust_helper_mmgrab(struct mm_struct *mm)
+{
+ mmgrab(mm);
+}
+
+void rust_helper_mmdrop(struct mm_struct *mm)
+{
+ mmdrop(mm);
+}
+
+void rust_helper_mmget(struct mm_struct *mm)
+{
+ mmget(mm);
+}
+
+bool rust_helper_mmget_not_zero(struct mm_struct *mm)
+{
+ return mmget_not_zero(mm);
+}
+
+void rust_helper_mmap_read_lock(struct mm_struct *mm)
+{
+ mmap_read_lock(mm);
+}
+
+bool rust_helper_mmap_read_trylock(struct mm_struct *mm)
+{
+ return mmap_read_trylock(mm);
+}
+
+void rust_helper_mmap_read_unlock(struct mm_struct *mm)
+{
+ mmap_read_unlock(mm);
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 496ed32b0911a9fdbce5d26738b9cf7ef910b269..9cf35fbff356d2b12a104dbf0ecda96c168b185f 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -57,6 +57,7 @@
pub mod kunit;
pub mod list;
pub mod miscdevice;
+pub mod mm;
#[cfg(CONFIG_NET)]
pub mod net;
pub mod of;
diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
new file mode 100644
index 0000000000000000000000000000000000000000..2fb5f440af602cc9a89a040edaee60d88d54168b
--- /dev/null
+++ b/rust/kernel/mm.rs
@@ -0,0 +1,209 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Memory management.
+//!
+//! This module deals with managing the address space of userspace processes. Each process has an
+//! instance of [`Mm`], which keeps track of multiple VMAs (virtual memory areas). Each VMA
+//! corresponds to a region of memory that the userspace process can access, and the VMA lets you
+//! control what happens when userspace reads or writes to that region of memory.
+//!
+//! C header: [`include/linux/mm.h`](srctree/include/linux/mm.h)
+
+use crate::{
+ bindings,
+ types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
+};
+use core::{ops::Deref, ptr::NonNull};
+
+/// A wrapper for the kernel's `struct mm_struct`.
+///
+/// This represents the address space of a userspace process, so each process has one `Mm`
+/// instance. It may hold many VMAs internally.
+///
+/// There is a counter called `mm_users` that counts the users of the address space; this includes
+/// the userspace process itself, but can also include kernel threads accessing the address space.
+/// Once `mm_users` reaches zero, this indicates that the address space can be destroyed. To access
+/// the address space, you must prevent `mm_users` from reaching zero while you are accessing it.
+/// The [`MmWithUser`] type represents an address space where this is guaranteed, and you can
+/// create one using [`mmget_not_zero`].
+///
+/// The `ARef<Mm>` smart pointer holds an `mmgrab` refcount. Its destructor may sleep.
+///
+/// # Invariants
+///
+/// Values of this type are always refcounted using `mmgrab`.
+///
+/// [`mmget_not_zero`]: Mm::mmget_not_zero
+#[repr(transparent)]
+pub struct Mm {
+ mm: Opaque<bindings::mm_struct>,
+}
+
+// SAFETY: It is safe to call `mmdrop` on another thread than where `mmgrab` was called.
+unsafe impl Send for Mm {}
+// SAFETY: All methods on `Mm` can be called in parallel from several threads.
+unsafe impl Sync for Mm {}
+
+// SAFETY: By the type invariants, this type is always refcounted.
+unsafe impl AlwaysRefCounted for Mm {
+ #[inline]
+ fn inc_ref(&self) {
+ // SAFETY: The pointer is valid since self is a reference.
+ unsafe { bindings::mmgrab(self.as_raw()) };
+ }
+
+ #[inline]
+ unsafe fn dec_ref(obj: NonNull<Self>) {
+ // SAFETY: The caller is giving up their refcount.
+ unsafe { bindings::mmdrop(obj.cast().as_ptr()) };
+ }
+}
+
+/// A wrapper for the kernel's `struct mm_struct`.
+///
+/// This type is like [`Mm`], but with non-zero `mm_users`. It can only be used when `mm_users` can
+/// be proven to be non-zero at compile-time, usually because the relevant code holds an `mmget`
+/// refcount. It can be used to access the associated address space.
+///
+/// The `ARef<MmWithUser>` smart pointer holds an `mmget` refcount. Its destructor may sleep.
+///
+/// # Invariants
+///
+/// Values of this type are always refcounted using `mmget`. The value of `mm_users` is non-zero.
+#[repr(transparent)]
+pub struct MmWithUser {
+ mm: Mm,
+}
+
+// SAFETY: It is safe to call `mmput` on another thread than where `mmget` was called.
+unsafe impl Send for MmWithUser {}
+// SAFETY: All methods on `MmWithUser` can be called in parallel from several threads.
+unsafe impl Sync for MmWithUser {}
+
+// SAFETY: By the type invariants, this type is always refcounted.
+unsafe impl AlwaysRefCounted for MmWithUser {
+ #[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(obj.cast().as_ptr()) };
+ }
+}
+
+// Make all `Mm` methods available on `MmWithUser`.
+impl Deref for MmWithUser {
+ type Target = Mm;
+
+ #[inline]
+ fn deref(&self) -> &Mm {
+ &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`.
+ #[inline]
+ pub fn as_raw(&self) -> *mut bindings::mm_struct {
+ self.mm.get()
+ }
+
+ /// Obtain a reference from a raw pointer.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `ptr` points at an `mm_struct`, and that it is not deallocated
+ /// during the lifetime 'a.
+ #[inline]
+ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a Mm {
+ // SAFETY: Caller promises that the pointer is valid for 'a. Layouts are compatible due to
+ // repr(transparent).
+ unsafe { &*ptr.cast() }
+ }
+
+ /// Calls `mmget_not_zero` and returns a handle if it succeeds.
+ #[inline]
+ pub fn mmget_not_zero(&self) -> Option<ARef<MmWithUser>> {
+ // SAFETY: The pointer is valid since self is a reference.
+ let success = unsafe { bindings::mmget_not_zero(self.as_raw()) };
+
+ if success {
+ // SAFETY: We just created an `mmget` refcount.
+ Some(unsafe { ARef::from_raw(NonNull::new_unchecked(self.as_raw().cast())) })
+ } else {
+ None
+ }
+ }
+}
+
+// These methods require `mm_users` to be non-zero.
+impl MmWithUser {
+ /// Obtain a reference from a raw pointer.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `ptr` points at an `mm_struct`, and that `mm_users` remains
+ /// non-zero for the duration of the lifetime 'a.
+ #[inline]
+ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a MmWithUser {
+ // SAFETY: Caller promises that the pointer is valid for 'a. The layout is compatible due
+ // to repr(transparent).
+ unsafe { &*ptr.cast() }
+ }
+
+ /// Lock the mmap read lock.
+ #[inline]
+ pub fn mmap_read_lock(&self) -> MmapReadGuard<'_> {
+ // SAFETY: The pointer is valid since self is a reference.
+ unsafe { bindings::mmap_read_lock(self.as_raw()) };
+
+ // INVARIANT: We just acquired the read lock.
+ MmapReadGuard {
+ mm: self,
+ _nts: NotThreadSafe,
+ }
+ }
+
+ /// Try to lock the mmap read lock.
+ #[inline]
+ pub fn mmap_read_trylock(&self) -> Option<MmapReadGuard<'_>> {
+ // SAFETY: The pointer is valid since self is a reference.
+ let success = unsafe { bindings::mmap_read_trylock(self.as_raw()) };
+
+ if success {
+ // INVARIANT: We just acquired the read lock.
+ Some(MmapReadGuard {
+ mm: self,
+ _nts: NotThreadSafe,
+ })
+ } else {
+ None
+ }
+ }
+}
+
+/// A guard for the mmap read lock.
+///
+/// # Invariants
+///
+/// This `MmapReadGuard` guard owns the mmap read lock.
+pub struct MmapReadGuard<'a> {
+ mm: &'a MmWithUser,
+ // `mmap_read_lock` and `mmap_read_unlock` must be called on the same thread
+ _nts: NotThreadSafe,
+}
+
+impl Drop for MmapReadGuard<'_> {
+ #[inline]
+ fn drop(&mut self) {
+ // SAFETY: We hold the read lock by the type invariants.
+ unsafe { bindings::mmap_read_unlock(self.mm.as_raw()) };