linux-kernel - Re: [PATCH v4 2/2] rust: zpool: add abstraction for zpool drivers

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Message-ID: <d91e6483-c5a6-468f-b40e-1acdd28a80cd@konsulko.se>
Date: Thu, 28 Aug 2025 12:01:06 +0200
From: Vitaly Wool <vitaly.wool@...sulko.se>
To: Danilo Krummrich <dakr@...nel.org>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
 Uladzislau Rezki <urezki@...il.com>, Alice Ryhl <aliceryhl@...gle.com>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Miguel Ojeda
 <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Bjorn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin
 <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>, Johannes Weiner <hannes@...xchg.org>,
 Yosry Ahmed <yosry.ahmed@...ux.dev>, Nhat Pham <nphamcs@...il.com>,
 linux-mm@...ck.org
Subject: Re: [PATCH v4 2/2] rust: zpool: add abstraction for zpool drivers

<snip>
>>> +    extern "C" fn destroy_(pool: *mut c_void) {
>>> +        // SAFETY: The pointer originates from an `into_foreign` call.
>>> +        T::destroy(unsafe { T::Pool::from_foreign(pool) })
>>> +    }
>>> +    extern "C" fn malloc_(
>>> +        pool: *mut c_void,
>>> +        size: usize,
>>> +        gfp: u32,
>>> +        handle: *mut usize,
>>> +        nid: c_int,
>>> +    ) -> c_int {
>>> +        // SAFETY: The pointer originates from an `into_foreign` 
>>> call. If `pool` is passed to
>>> +        // `from_foreign`, then that happens in `_destroy` which 
>>> will not be called during this
>>> +        // method.
>>> +        let pool = unsafe { T::Pool::borrow_mut(pool) };
>>
>> Wait, can't this happen concurrently to all the other functions that 
>> borrow the
>> pool? This would be undefined behavior, no?
> 
> Theoretically, yes, but since pool is actually Box<T>, it's only the 
> inner T that is mutable.
> 
> Anyway, the only reason for malloc() to require a mutable reference is 
> that the backend implementation *may* use RBTree::cursor_lower_bound() 
> which requires a mutable reference of the tree.
> 
> Would it be okay if I
> * change the Zpool API so that malloc takes an immutable reference
> * extend the RBTree API with a cursor_lower_bound analog which doesn't 
> require a mutable tree?

As a matter of fact, the RBTree change may be postponed and submitted 
together with zblock, it is not relevant until then.

~Vitaly