linux-kernel - Re: [PATCH v8 5/7] mm: rust: add mmput

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Message-ID: <5762ab92-3091-46cf-9565-f59cf917b470@lucifer.local>
Date: Wed, 20 Nov 2024 19:46:46 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, John Hubbard <jhubbard@...dia.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rust-for-linux@...r.kernel.org,
        Andreas Hindborg <a.hindborg@...nel.org>
Subject: Re: [PATCH v8 5/7] mm: rust: add mmput_async support

On Wed, Nov 20, 2024 at 02:49:59PM +0000, Alice Ryhl 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.

Oh Lord, I didn't even know this existed... I see it was (re-)added in commit
a1b2289cef92 ("android: binder: drop lru lock in isolate callback") back in 2017
so quite a history of being necessary for binder.

I see mmdrop_async(), I guess that's not needed for anything binder-ish? A quick
look in the code suggests this is invoked in free_signal_struct() and is there
due to some softirq stuff on x86... so yeah I guess not :)

>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/mm.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
>
> diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
> index a15acb546f68..f800b6c244cd 100644
> --- a/rust/kernel/mm.rs
> +++ b/rust/kernel/mm.rs
> @@ -98,6 +98,48 @@ 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 {
>      /// Call `mmgrab` on `current.mm`.
> @@ -171,6 +213,13 @@ 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 use_mmput_async(me: ARef<MmWithUser>) -> ARef<MmWithUserAsync> {

Again, nitty, but I wonder if this should be as_xxx()?

But I guess this makes sense too.

> +        // SAFETY: The layouts and invariants are compatible.
> +        unsafe { ARef::from_raw(ARef::into_raw(me).cast()) }
> +    }
> +
>      /// Try to lock the vma read lock under rcu.
>      ///
>      /// If this operation fails, the vma may still exist. In that case, you should take the mmap
>
> --
> 2.47.0.371.ga323438b13-goog
>