`struct page` is managed by a native reference count, and kernel C code routinely takes its own references to pages, for example when a page is inserted into a VMA with `vm_insert_page()`.
Thus, implement `RefCounted` for `Page`, backed by `get_page()` and `put_page()`, mark it `AlwaysRefCounted`, and return `ARef<Page>` from `Page::alloc_page()`. The page is freed when the last reference to it is dropped; for the order-0 pages allocated here, this is equivalent to `__free_pages()`. This also allows `Page` references to be returned as borrowed references without owning the `struct page`. Remove `BorrowedPage` and update users to use `&Page` and `ARef<Page>`. Assisted-by: LLM Suggested-by: Alice Ryhl <[email protected]> Signed-off-by: Andreas Hindborg <[email protected]> --- drivers/android/binder/page_range.rs | 8 +- rust/helpers/page.c | 10 +++ rust/kernel/alloc/allocator.rs | 21 +++--- rust/kernel/alloc/allocator/iter.rs | 6 +- rust/kernel/page.rs | 139 ++++++++++++----------------------- 5 files changed, 73 insertions(+), 111 deletions(-) diff --git a/drivers/android/binder/page_range.rs b/drivers/android/binder/page_range.rs index 52ffbf3504e7f..1117e1f6ece02 100644 --- a/drivers/android/binder/page_range.rs +++ b/drivers/android/binder/page_range.rs @@ -198,7 +198,7 @@ unsafe impl Send for Inner {} #[repr(C)] struct PageInfo { lru: bindings::list_head, - page: Option<Page>, + page: Option<ARef<Page>>, range: *const ShrinkablePageRange, } @@ -206,7 +206,7 @@ impl PageInfo { /// # Safety /// /// The caller ensures that writing to `me.page` is ok, and that the page is not currently set. - unsafe fn set_page(me: *mut PageInfo, page: Page) { + unsafe fn set_page(me: *mut PageInfo, page: ARef<Page>) { // SAFETY: This pointer offset is in bounds. let ptr = unsafe { &raw mut (*me).page }; @@ -229,13 +229,13 @@ unsafe fn get_page<'a>(me: *const PageInfo) -> Option<&'a Page> { let ptr = unsafe { &raw const (*me).page }; // SAFETY: The pointer is valid for reading. - unsafe { (*ptr).as_ref() } + unsafe { (*ptr).as_deref() } } /// # Safety /// /// The caller ensures that writing to `me.page` is ok for the duration of 'a. - unsafe fn take_page(me: *mut PageInfo) -> Option<Page> { + unsafe fn take_page(me: *mut PageInfo) -> Option<ARef<Page>> { // SAFETY: This pointer offset is in bounds. let ptr = unsafe { &raw mut (*me).page }; diff --git a/rust/helpers/page.c b/rust/helpers/page.c index f8463fbed2a26..906eb4b18f029 100644 --- a/rust/helpers/page.c +++ b/rust/helpers/page.c @@ -10,6 +10,16 @@ __rust_helper struct page *rust_helper_alloc_pages(gfp_t gfp_mask, return alloc_pages(gfp_mask, order); } +__rust_helper void rust_helper_get_page(struct page *page) +{ + get_page(page); +} + +__rust_helper void rust_helper_put_page(struct page *page) +{ + put_page(page); +} + __rust_helper void *rust_helper_kmap_local_page(struct page *page) { return kmap_local_page(page); diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs index cd4203f27aed0..27d957b7956e5 100644 --- a/rust/kernel/alloc/allocator.rs +++ b/rust/kernel/alloc/allocator.rs @@ -17,7 +17,7 @@ use crate::{ bindings, - page, // + page::Page, // }; use core::{ @@ -169,7 +169,7 @@ unsafe fn realloc( } impl Vmalloc { - /// Convert a pointer to a [`Vmalloc`] allocation to a [`page::BorrowedPage`]. + /// Convert a pointer to a [`Vmalloc`] allocation to a [`Page`] reference. /// /// # Examples /// @@ -202,20 +202,17 @@ impl Vmalloc { /// /// - `ptr` must be a valid pointer to a [`Vmalloc`] allocation. /// - `ptr` must remain valid for the entire duration of `'a`. - pub unsafe fn to_page<'a>(ptr: NonNull<u8>) -> page::BorrowedPage<'a> { + pub unsafe fn to_page<'a>(ptr: NonNull<u8>) -> &'a Page { // SAFETY: `ptr` is a valid pointer to `Vmalloc` memory. let page = unsafe { bindings::vmalloc_to_page(ptr.as_ptr().cast()) }; - // SAFETY: `vmalloc_to_page` returns a valid pointer to a `struct page` for a valid pointer - // to `Vmalloc` memory. - let page = unsafe { NonNull::new_unchecked(page) }; - // SAFETY: - // - `page` is a valid pointer to a `struct page`, given that by the safety requirements of - // this function `ptr` is a valid pointer to a `Vmalloc` allocation. - // - By the safety requirements of this function `ptr` is valid for the entire lifetime of - // `'a`. - unsafe { page::BorrowedPage::from_raw(page) } + // - `vmalloc_to_page` returns a valid, non-null pointer to a `struct page` for a valid + // pointer to `Vmalloc` memory, given that by the safety requirements of this function + // `ptr` is a valid pointer to a `Vmalloc` allocation. + // - By the safety requirements of this function `ptr`, and hence the `struct page`, is + // valid for the entire lifetime of `'a`. + unsafe { &*page.cast() } } } diff --git a/rust/kernel/alloc/allocator/iter.rs b/rust/kernel/alloc/allocator/iter.rs index 02fda3ea5cae6..8dcc16ed89893 100644 --- a/rust/kernel/alloc/allocator/iter.rs +++ b/rust/kernel/alloc/allocator/iter.rs @@ -9,7 +9,7 @@ ptr::NonNull, // }; -/// An [`Iterator`] of [`page::BorrowedPage`] items owned by a [`Vmalloc`] allocation. +/// An [`Iterator`] of [`Page`](page::Page) references owned by a [`Vmalloc`] allocation. /// /// # Guarantees /// @@ -28,11 +28,11 @@ pub struct VmallocPageIter<'a> { size: usize, /// The current page index of the [`Iterator`]. index: usize, - _p: PhantomData<page::BorrowedPage<'a>>, + _p: PhantomData<&'a page::Page>, } impl<'a> Iterator for VmallocPageIter<'a> { - type Item = page::BorrowedPage<'a>; + type Item = &'a page::Page; fn next(&mut self) -> Option<Self::Item> { let offset = self.index.checked_mul(page::PAGE_SIZE)?; diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs index 1c0796ea229f0..a2610774c7499 100644 --- a/rust/kernel/page.rs +++ b/rust/kernel/page.rs @@ -12,16 +12,17 @@ code::*, Result, // }, + sync::aref::{ + ARef, + AlwaysRefCounted, + RefCounted, // + }, + types::Opaque, uaccess::UserSliceReader, // }; -use core::{ - marker::PhantomData, - mem::ManuallyDrop, - ops::Deref, - ptr::{ - self, - NonNull, // - }, // +use core::ptr::{ + self, + NonNull, // }; /// A bitwise shift for the page size. @@ -65,93 +66,30 @@ pub const fn page_align(addr: usize) -> Option<usize> { Some(sum & PAGE_MASK) } -/// Representation of a non-owning reference to a [`Page`]. -/// -/// This type provides a borrowed version of a [`Page`] that is owned by some other entity, e.g. a -/// [`Vmalloc`] allocation such as [`VBox`]. -/// -/// # Example -/// -/// ``` -/// # use kernel::{bindings, prelude::*}; -/// use kernel::page::{BorrowedPage, Page, PAGE_SIZE}; -/// # use core::{mem::MaybeUninit, ptr, ptr::NonNull }; -/// -/// fn borrow_page<'a>(vbox: &'a mut VBox<MaybeUninit<[u8; PAGE_SIZE]>>) -> BorrowedPage<'a> { -/// let ptr = ptr::from_ref(&**vbox); -/// -/// // SAFETY: `ptr` is a valid pointer to `Vmalloc` memory. -/// let page = unsafe { bindings::vmalloc_to_page(ptr.cast()) }; -/// -/// // SAFETY: `vmalloc_to_page` returns a valid pointer to a `struct page` for a valid -/// // pointer to `Vmalloc` memory. -/// let page = unsafe { NonNull::new_unchecked(page) }; -/// -/// // SAFETY: -/// // - `self.0` is a valid pointer to a `struct page`. -/// // - `self.0` is valid for the entire lifetime of `self`. -/// unsafe { BorrowedPage::from_raw(page) } -/// } -/// -/// let mut vbox = VBox::<[u8; PAGE_SIZE]>::new_uninit(GFP_KERNEL)?; -/// let page = borrow_page(&mut vbox); -/// -/// // SAFETY: There is no concurrent read or write to this page. -/// unsafe { page.fill_zero_raw(0, PAGE_SIZE)? }; -/// # Ok::<(), Error>(()) -/// ``` -/// -/// # Invariants -/// -/// The borrowed underlying pointer to a `struct page` is valid for the entire lifetime `'a`. -/// -/// [`VBox`]: kernel::alloc::VBox -/// [`Vmalloc`]: kernel::alloc::allocator::Vmalloc -pub struct BorrowedPage<'a>(ManuallyDrop<Page>, PhantomData<&'a Page>); - -impl<'a> BorrowedPage<'a> { - /// Constructs a [`BorrowedPage`] from a raw pointer to a `struct page`. - /// - /// # Safety - /// - /// - `ptr` must point to a valid `bindings::page`. - /// - `ptr` must remain valid for the entire lifetime `'a`. - pub unsafe fn from_raw(ptr: NonNull<bindings::page>) -> Self { - let page = Page { page: ptr }; - - // INVARIANT: The safety requirements guarantee that `ptr` is valid for the entire lifetime - // `'a`. - Self(ManuallyDrop::new(page), PhantomData) - } -} - -impl<'a> Deref for BorrowedPage<'a> { - type Target = Page; - - fn deref(&self) -> &Self::Target { - &self.0 - } -} - -/// Trait to be implemented by types which provide an [`Iterator`] implementation of -/// [`BorrowedPage`] items, such as [`VmallocPageIter`](kernel::alloc::allocator::VmallocPageIter). +/// Trait to be implemented by types which provide an [`Iterator`] of [`Page`] references, such as +/// [`VmallocPageIter`](kernel::alloc::allocator::VmallocPageIter). pub trait AsPageIter { /// The [`Iterator`] type, e.g. [`VmallocPageIter`](kernel::alloc::allocator::VmallocPageIter). - type Iter<'a>: Iterator<Item = BorrowedPage<'a>> + type Iter<'a>: Iterator<Item = &'a Page> where Self: 'a; - /// Returns an [`Iterator`] of [`BorrowedPage`] items over all pages owned by `self`. + /// Returns an [`Iterator`] of [`Page`] references over all pages owned by `self`. fn page_iter(&mut self) -> Self::Iter<'_>; } -/// A pointer to a page that owns the page allocation. +/// A `struct page`. +/// +/// A `Page` is accessed through a shared reference or through an owning [`ARef<Page>`]. The page +/// allocation is freed when the last reference to it is dropped. /// /// # Invariants /// -/// The pointer is valid, and has ownership over the page. +/// The `Page` is backed by a valid `struct page` whose allocation is managed by the page +/// reference count. +#[repr(transparent)] pub struct Page { - page: NonNull<bindings::page>, + page: Opaque<bindings::page>, } // SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across @@ -185,19 +123,20 @@ impl Page { /// # Ok::<(), kernel::alloc::AllocError>(()) /// ``` #[inline] - pub fn alloc_page(flags: Flags) -> Result<Self, AllocError> { + pub fn alloc_page(flags: Flags) -> Result<ARef<Self>, AllocError> { // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it // is always safe to call this method. let page = unsafe { bindings::alloc_pages(flags.as_raw(), 0) }; let page = NonNull::new(page).ok_or(AllocError)?; - // INVARIANT: We just successfully allocated a page, so we now have ownership of the newly - // allocated page. We transfer that ownership to the new `Page` object. - Ok(Self { page }) + // SAFETY: We just successfully allocated a page, so we own the reference count returned + // by `alloc_pages`, and we transfer that reference to the new `ARef<Page>`. Since `Page` + // is transparent, we can cast the pointer directly. + Ok(unsafe { ARef::from_raw(page.cast()) }) } /// Returns a raw pointer to the page. pub fn as_ptr(&self) -> *mut bindings::page { - self.page.as_ptr() + self.page.get() } /// Get the node id containing this page. @@ -373,10 +312,26 @@ pub unsafe fn copy_from_user_slice_raw( } } -impl Drop for Page { +// SAFETY: `struct page` is managed by the page reference count. Incrementing it via `get_page()` +// keeps the page alive at least until a matching `put_page()`, which frees the page when the +// count reaches zero. By the type invariant, all `Page` instances are backed by refcounted +// pages. +unsafe impl RefCounted for Page { #[inline] - fn drop(&mut self) { - // SAFETY: By the type invariants, we have ownership of the page and can free it. - unsafe { bindings::__free_pages(self.page.as_ptr(), 0) }; + fn inc_ref(&self) { + // SAFETY: By the type invariant, `self.as_ptr()` points to a valid `struct page` with a + // non-zero reference count. + unsafe { bindings::get_page(self.as_ptr()) }; + } + + #[inline] + unsafe fn dec_ref(obj: NonNull<Self>) { + // SAFETY: By the function safety requirements, the caller relinquishes a reference count + // on the page. Since `Page` is transparent, we can cast the pointer directly. + unsafe { bindings::put_page(obj.cast().as_ptr()) }; } } + +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Page>` from a +// `&Page`. +unsafe impl AlwaysRefCounted for Page {} -- 2.51.2
