linux-kernel - Re: [PATCH v16 8/9] task: rust: rework how current is accessed

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Message-ID: <aBuOX0hTLZed3JND@Mac.home>
Date: Wed, 7 May 2025 09:46:23 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	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>, Jann Horn <jannh@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v16 8/9] task: rust: rework how current is accessed

On Wed, May 07, 2025 at 09:06:01AM -0700, Boqun Feng wrote:
[...]
> > +impl CurrentTask {
> > +    /// Access the address space of the current task.
> > +    ///
> > +    /// This function does not touch the refcount of the mm.
> > +    #[inline]
> > +    pub fn mm(&self) -> Option<&MmWithUser> {
> > +        // SAFETY: The `mm` field of `current` is not modified from other threads, so reading it is
> > +        // not a data race.
> > +        let mm = unsafe { (*self.as_ptr()).mm };
> > +
> > +        if mm.is_null() {
> > +            return None;
> > +        }
> > +
> > +        // SAFETY: If `current->mm` is non-null, then it references a valid mm with a non-zero
> > +        // value of `mm_users`. Furthermore, the returned `&MmWithUser` borrows from this
> > +        // `CurrentTask`, so it cannot escape the scope in which the current pointer was obtained.
> > +        //
> > +        // This is safe even if `kthread_use_mm()`/`kthread_unuse_mm()` are used. There are two
> > +        // relevant cases:
> > +        // * If the `&CurrentTask` was created before `kthread_use_mm()`, then it cannot be
> > +        //   accessed during the `kthread_use_mm()`/`kthread_unuse_mm()` scope due to the
> > +        //   `NotThreadSafe` field of `CurrentTask`.
> > +        // * If the `&CurrentTask` was created within a `kthread_use_mm()`/`kthread_unuse_mm()`
> > +        //   scope, then the `&CurrentTask` cannot escape that scope, so the returned `&MmWithUser`
> > +        //   also cannot escape that scope.
> > +        // In either case, it's not possible to read `current->mm` and keep using it after the
> > +        // scope is ended with `kthread_unuse_mm()`.
> > +        Some(unsafe { MmWithUser::from_raw(mm) })
> > +    }
> > +
> 
> Due to this:
> 
> 	https://lore.kernel.org/oe-kbuild-all/202505072116.eSYC8igT-lkp@intel.com/
> 
> , I think we should move this impl block into rust/kernel/mm.rs for now,
> i.e. (in rust/kernel/mm.rs)
> 
>     impl crate::task::CurrentTask {
>         pub fn mm(&self) -> Option<&MmWithUser> { ... }
>     }
> 
> Thoughts?
> 

Hmm.. this alone won't be enough, because miscdevice also uses mm. Maybe
you could most of mm defined even when CONFIG_MMU=n but keep
MmWithUserAsync only available when CONFIG_MMU=y?

Regards,
Boqun

> Regards,
> Boqun
> 
[...]