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Howlett" , Andrew Morton , Greg Kroah-Hartman , Arnd Bergmann , Jann Horn , Suren Baghdasaryan , Alex Gaynor , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Andreas Hindborg , Trevor Gross , linux-kernel@vger.kernel.org, linux-mm@kvack.org, rust-for-linux@vger.kernel.org Subject: Re: [PATCH v16 8/9] task: rust: rework how current is accessed Message-ID: References: <20250408-vma-v16-0-d8b446e885d9@google.com> <20250408-vma-v16-8-d8b446e885d9@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: EA6FAA0007 X-Rspamd-Server: rspam04 X-Stat-Signature: 9uu7k8izidhoy337js13b58qcoi1w3y8 X-HE-Tag: 1746636942-66352 X-HE-Meta: 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 ekcvYH4S 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, May 07, 2025 at 09:46:23AM -0700, Boqun Feng wrote: > 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? > Something like this, probably? But your choice ;-) Make CONFIG_RUST select CONFIG_MMU is fine but the question is who is going to unselect that at when? Regards, Boqun ------------------>8 diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs index 615907a0f3b4..8a1c8158f2b3 100644 --- a/rust/kernel/mm.rs +++ b/rust/kernel/mm.rs @@ -10,7 +10,6 @@ //! control what happens when userspace reads or writes to that region of memory. //! //! C header: [`include/linux/mm.h`](srctree/include/linux/mm.h) -#![cfg(CONFIG_MMU)] use crate::{ bindings, @@ -130,6 +129,7 @@ unsafe impl Send for MmWithUserAsync {} // SAFETY: All methods on `MmWithUserAsync` can be called in parallel from several threads. unsafe impl Sync for MmWithUserAsync {} +#[cfg(CONFIG_MMU)] // SAFETY: By the type invariants, this type is always refcounted. unsafe impl AlwaysRefCounted for MmWithUserAsync { #[inline] @@ -207,6 +207,7 @@ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a MmWithUser { } /// Use `mmput_async` when dropping this refcount. + #[cfg(CONFIG_MMU)] #[inline] pub fn into_mmput_async(me: ARef) -> ARef { // SAFETY: The layouts and invariants are compatible.