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

[Boqun Feng <boqun.feng@gmail.com>]

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
> 
[...]