UPSTREAM: mm: rust: add mmput_async support

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
9a9ab0d963 ("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.

Link: https://lkml.kernel.org/r/20250408-vma-v16-5-d8b446e885d9@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Benno Lossin <benno.lossin@proton.me>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jann Horn <jannh@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Trevor Gross <tmgross@umich.edu>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Bug: 429146594
(cherry picked from commit 114ba9b9e8191925438365089c0d300f4f1b6fb1)
Change-Id: Iad1464e84163d33320d9ce1e87f5e29f16da5dae
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

rust/kernel/mm.rs

@@ -111,6 +111,50 @@ impl Deref for MmWithUser {
     }
 }
 
+/// 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`.
@@ -162,6 +206,13 @@ impl 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