Adds an MmWithUserAsync type that uses mmput_async when dropped but is
otherwise identical to MmWithUser. This has to be done using a separate
type because the thing we are changing is the destructor.

Rust Binder needs this to avoid a certain deadlock. See commit
9a9ab0d96362 ("binder: fix race between mmput() and do_exit()") for
details. It's also needed in the shrinker to avoid cleaning up the mm in
the shrinker's context.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> (for mm bits)
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/mm.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
index 618aa48e00a4..42decd311740 100644
--- a/rust/kernel/mm.rs
+++ b/rust/kernel/mm.rs
@@ -110,6 +110,48 @@ 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 {
+    fn inc_ref(&self) {
+        // SAFETY: The pointer is valid since self is a reference.
+        unsafe { bindings::mmget(self.as_raw()) };
+    }
+
+    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`.
@@ -161,6 +203,13 @@ 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

-- 
2.48.1.502.g6dc24dfdaf-goog

On Thu, 13 Feb 2025 11:04:04 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> Adds an MmWithUserAsync type that uses mmput_async when dropped but is
> otherwise identical to MmWithUser. This has to be done using a separate
> type because the thing we are changing is the destructor.
> 
> Rust Binder needs this to avoid a certain deadlock. See commit
> 9a9ab0d96362 ("binder: fix race between mmput() and do_exit()") for
> details. It's also needed in the shrinker to avoid cleaning up the mm in
> the shrinker's context.
> 
> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
> Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> (for mm bits)
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/mm.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
> 
> diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
> index 618aa48e00a4..42decd311740 100644
> --- a/rust/kernel/mm.rs
> +++ b/rust/kernel/mm.rs
> @@ -110,6 +110,48 @@ 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 {
> +    fn inc_ref(&self) {
> +        // SAFETY: The pointer is valid since self is a reference.
> +        unsafe { bindings::mmget(self.as_raw()) };
> +    }
> +
> +    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`.
> @@ -161,6 +203,13 @@ 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
>