linux-kernel - Re: [PATCH] rust: mm: add abstractions for mm_struct and vm_area

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Message-ID: <324d3628-a537-431f-8dd4-f33184600537@proton.me>
Date: Fri, 26 Jul 2024 08:26:46 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] rust: mm: add abstractions for mm_struct and vm_area_struct

On 26.07.24 10:14, Alice Ryhl wrote:
> On Fri, Jul 26, 2024 at 10:11 AM Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> On 23.07.24 16:32, Alice Ryhl wrote:
>>> +pub struct MmGet {
>>> +    mm: NonNull<bindings::mm_struct>,
>>> +}
>>> +
>>> +impl MmGet {
>>> +    /// Lock the mmap read lock.
>>> +    #[inline]
>>> +    pub fn mmap_write_lock(&self) -> MmapWriteLock<'_> {
>>> +        // SAFETY: The pointer is valid since we hold a refcount.
>>> +        unsafe { bindings::mmap_write_lock(self.mm.as_ptr()) };
>>> +
>>> +        // INVARIANT: We just acquired the write lock, so we can transfer to this guard.
>>> +        //
>>> +        // The signature of this function ensures that the `MmapWriteLock` will not outlive this
>>> +        // `mmget` refcount.
>>> +        MmapWriteLock {
>>> +            mm: self.mm,
>>> +            _lifetime: PhantomData,
>>> +        }
>>> +    }
>>> +
>>> +    /// When dropping this refcount, use `mmput_async` instead of `mmput`.
>>
>> I don't get this comment.
> 
> The C side provides two ways to decrement the mmget refcount. One is
> mmput and the other is mmput_async. The difference is that when the
> refcount hits zero, mmput_async cleans up the mm_struct on the
> workqueue, whereas mmput cleans it up immediately. This means that
> mmput_async is safe in atomic context, but mmput is not.

I see, IMO this would be a better comment:

/// Converts to this `MmGet` to `MmGetAsync`.
///
/// `MmGetAsync` uses `mmput_async` instead of `mmput` for decrementing
/// the refcount.

Since from a Rust perspective, this is just a conversion function. Maybe
the name should also reflect that ie `to_mm_get_async` or similar.

---
Cheers,
Benno