[PATCH v13 2/2] rust: xarray: Add an abstraction for XArray

Tamir Duberstein
2024-12-13T20:02:09Z

`XArray` is an efficient sparse array of pointers. Add a Rust
abstraction for this type.

This implementation bounds the element type on `ForeignOwnable` and
requires explicit locking for all operations. Future work may leverage
RCU to enable lockless operation.

Inspired-by: Maíra Canal <mcanal@igalia.com>
Inspired-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
rust/bindings/bindings_helper.h | 6 +
rust/helpers/helpers.c | 1 +
rust/helpers/xarray.c | 28 ++++
rust/kernel/alloc.rs | 5 +
rust/kernel/lib.rs | 1 +
rust/kernel/xarray.rs | 289 ++++++++++++++++++++++++++++++++++++++++
6 files changed, 330 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 5c4dfe22f41a5a106330e8c43ffbd342c69c4e0b..9f39d673b240281aed2759b5bd076aa43fb54951 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -30,6 +30,7 @@
#include <linux/tracepoint.h>
#include <linux/wait.h>
#include <linux/workqueue.h>
+#include <linux/xarray.h>
#include <trace/events/rust_sample.h>

/* `bindgen` gets confused at certain things. */
@@ -43,3 +44,8 @@ const gfp_t RUST_CONST_HELPER___GFP_ZERO = __GFP_ZERO;
const gfp_t RUST_CONST_HELPER___GFP_HIGHMEM = ___GFP_HIGHMEM;
const gfp_t RUST_CONST_HELPER___GFP_NOWARN = ___GFP_NOWARN;
const blk_features_t RUST_CONST_HELPER_BLK_FEAT_ROTATIONAL = BLK_FEAT_ROTATIONAL;
+
+const xa_mark_t RUST_CONST_HELPER_XA_PRESENT = XA_PRESENT;
+
+const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC = XA_FLAGS_ALLOC;
+const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC1 = XA_FLAGS_ALLOC1;
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index dcf827a61b52e71e46fd5378878602eef5e538b6..ff28340e78c53c79baf18e2927cc90350d8ab513 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -30,3 +30,4 @@
#include "vmalloc.c"
#include "wait.c"
#include "workqueue.c"
+#include "xarray.c"
diff --git a/rust/helpers/xarray.c b/rust/helpers/xarray.c
new file mode 100644
index 0000000000000000000000000000000000000000..60b299f11451d2c4a75e50e25dec4dac13f143f4
--- /dev/null
+++ b/rust/helpers/xarray.c
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/xarray.h>
+
+int rust_helper_xa_err(void *entry)
+{
+ return xa_err(entry);
+}
+
+void rust_helper_xa_init_flags(struct xarray *xa, gfp_t flags)
+{
+ return xa_init_flags(xa, flags);
+}
+
+int rust_helper_xa_trylock(struct xarray *xa)
+{
+ return xa_trylock(xa);
+}
+
+void rust_helper_xa_lock(struct xarray *xa)
+{
+ return xa_lock(xa);
+}
+
+void rust_helper_xa_unlock(struct xarray *xa)
+{
+ return xa_unlock(xa);
+}
diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index f2f7f3a53d298cf899e062346202ba3285ce3676..be9f164ece2e0fe71143e0201247d2b70c193c51 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -39,6 +39,11 @@
pub struct Flags(u32);

impl Flags {
+ /// Get a flags value with all bits unset.
+ pub fn empty() -> Self {
+ Self(0)
+ }
+
/// Get the raw representation of this flag.
pub(crate) fn as_raw(self) -> u32 {
self.0
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index e1065a7551a39e68d6379031d80d4be336e652a3..9ca524b15920c525c7db419e60dec4c43522751d 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -68,6 +68,7 @@
pub mod types;
pub mod uaccess;
pub mod workqueue;
+pub mod xarray;

#[doc(hidden)]
pub use bindings;
diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
new file mode 100644
index 0000000000000000000000000000000000000000..e320198301e14413e9d54e8c457e399eeebcb4a4
--- /dev/null
+++ b/rust/kernel/xarray.rs
@@ -0,0 +1,289 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! XArray abstraction.
+//!
+//! C header: [`include/linux/xarray.h`](srctree/include/linux/xarray.h)
+
+use crate::{
+ alloc, bindings, build_assert, build_error,
+ error::{Error, Result},
+ init::PinInit,
+ pin_init,
+ types::{ForeignOwnable, NotThreadSafe, Opaque},
+};
+use core::{iter, marker::PhantomData, mem, pin::Pin, ptr::NonNull};
+use macros::{pin_data, pinned_drop};
+
+/// An array which efficiently maps sparse integer indices to owned objects.
+///
+/// This is similar to a [`crate::alloc::kvec::Vec<Option<T>>`], but more efficient when there are
+/// holes in the index space, and can be efficiently grown.
+///
+/// # Invariants
+///
+/// `self.xa` is always an initialized and valid [`bindings::xarray`] whose entries are either
+/// `XA_ZERO_ENTRY` or came from `T::into_foreign`.
+///
+/// # Examples
+///
+/// ```rust
+/// use kernel::alloc::KBox;
+/// use kernel::xarray::{AllocKind, XArray};
+///
+/// let xa = KBox::pin_init(XArray::new(AllocKind::Alloc1), GFP_KERNEL)?;
+///
+/// let dead = KBox::new(0xdead, GFP_KERNEL)?;
+/// let beef = KBox::new(0xbeef, GFP_KERNEL)?;
+///
+/// let mut guard = xa.lock();
+///
+/// assert_eq!(guard.get(0), None);
+///
+/// assert_eq!(guard.store(0, dead, GFP_KERNEL)?.as_deref(), None);
+/// assert_eq!(guard.get(0).copied(), Some(0xdead));
+///
+/// *guard.get_mut(0).unwrap() = 0xffff;
+/// assert_eq!(guard.get(0).copied(), Some(0xffff));
+///
+/// assert_eq!(guard.store(0, beef, GFP_KERNEL)?.as_deref().copied(), Some(0xffff));
+/// assert_eq!(guard.get(0).copied(), Some(0xbeef));
+///
+/// guard.remove(0);
+/// assert_eq!(guard.get(0), None);
+///
+/// # Ok::<(), Error>(())
+/// ```
+#[pin_data(PinnedDrop)]
+pub struct XArray<T: ForeignOwnable> {
+ #[pin]
+ xa: Opaque<bindings::xarray>,
+ _p: PhantomData<T>,
+}
+
+#[pinned_drop]
+impl<T: ForeignOwnable> PinnedDrop for XArray<T> {
+ fn drop(self: Pin<&mut Self>) {
+ self.iter().for_each(|ptr| {
+ let ptr = ptr.as_ptr();
+ // SAFETY: `ptr` came from `T::into_foreign`.
+ //
+ // INVARIANT: we own the only reference to the array which is being dropped so the
+ // broken invariant is not observable on function exit.
+ drop(unsafe { T::from_foreign(ptr) })
+ });
+
+ // SAFETY: `self.xa` is always valid by the type invariant.
+ unsafe { bindings::xa_destroy(self.xa.get()) };
+ }
+}
+
+/// Flags passed to [`XArray::new`] to configure the array's allocation tracking behavior.
+pub enum AllocKind {
+ /// Consider the first element to be at index 0.
+ Alloc,
+ /// Consider the first element to be at index 1.
+ Alloc1,
+}
+
+impl<T: ForeignOwnable> XArray<T> {
+ /// Creates a new [`XArray`].
+ pub fn new(kind: AllocKind) -> impl PinInit<Self> {
+ let flags = match kind {
+ AllocKind::Alloc => bindings::XA_FLAGS_ALLOC,
+ AllocKind::Alloc1 => bindings::XA_FLAGS_ALLOC1,
+ };
+ pin_init!(Self {
+ // SAFETY: `xa` is valid while the closure is called.
+ xa <- Opaque::ffi_init(|xa| unsafe {
+ bindings::xa_init_flags(xa, flags)
+ }),
+ _p: PhantomData,
+ })
+ }
+
+ fn iter(&self) -> impl Iterator<Item = NonNull<T::PointedTo>> + '_ {
+ // TODO: Remove and use usize::MAX when
+ // https://lore.kernel.org/all/20240913213041.395655-5-gary@garyguo.net/ is applied.
+ const MAX: crate::ffi::c_ulong = crate::ffi::c_ulong::MAX;
+
+ let mut index = 0;
+
+ // SAFETY: `self.xa` is always valid by the type invariant.
+ iter::once(unsafe {
+ bindings::xa_find(self.xa.get(), &mut index, MAX, bindings::XA_PRESENT)
+ })
+ .chain(iter::from_fn(move || {
+ // SAFETY: `self.xa` is always valid by the type invariant.
+ Some(unsafe {
+ bindings::xa_find_after(self.xa.get(), &mut index, MAX, bindings::XA_PRESENT)
+ })
+ }))
+ .map_while(|ptr| NonNull::new(ptr.cast()))
+ }
+
+ /// Attempts to lock the [`XArray`] for exclusive access.
+ pub fn try_lock(&self) -> Option<Guard<'_, T>> {
+ // SAFETY: `self.xa` is always valid by the type invariant.
+ if (unsafe { bindings::xa_trylock(self.xa.get()) } != 0) {
+ Some(Guard {
+ xa: self,
+ _not_send: NotThreadSafe,
+ })
+ } else {
+ None
+ }
+ }
+
+ /// Locks the [`XArray`] for exclusive access.
+ pub fn lock(&self) -> Guard<'_, T> {
+ // SAFETY: `self.xa` is always valid by the type invariant.
+ unsafe { bindings::xa_lock(self.xa.get()) };
+
+ Guard {
+ xa: self,
+ _not_send: NotThreadSafe,
+ }
+ }
+}
+
+/// A lock guard.
+///
+/// The lock is unlocked when the guard goes out of scope.
+#[must_use = "the lock unlocks immediately when the guard is unused"]
+pub struct Guard<'a, T: ForeignOwnable> {
+ xa: &'a XArray<T>,
+ _not_send: NotThreadSafe,
+}
+
+impl<T: ForeignOwnable> Drop for Guard<'_, T> {
+ fn drop(&mut self) {
+ // SAFETY:
+ // - `self.xa.xa` is always valid by the type invariant.
+ // - The caller holds the lock, so it is safe to unlock it.
+ unsafe { bindings::xa_unlock(self.xa.xa.get()) };
+ }
+}
+
+// TODO: Remove when https://lore.kernel.org/all/20240913213041.395655-5-gary@garyguo.net/ is applied.
+fn to_index(i: usize) -> crate::ffi::c_ulong {
+ i.try_into()
+ .unwrap_or_else(|_| build_error!("cannot convert usize to c_ulong"))
+}
+
+/// The error returned by [`store`](Guard::store).
+///
+/// Contains the underlying error and the value that was not stored.
+pub struct StoreError<T> {
+ /// The error that occurred.
+ pub error: Error,
+ /// The value that was not stored.
+ pub value: T,
+}
+
+impl<T> From<StoreError<T>> for Error {
+ fn from(value: StoreError<T>) -> Self {
+ let StoreError { error, value: _ } = value;
+ error
+ }
+}
+
+impl<'a, T: ForeignOwnable> Guard<'a, T> {
+ fn load<F, U>(&self, index: usize, f: F) -> Option
+ where
+ F: FnOnce(NonNull<T::PointedTo>) -> U,
+ {
+ // SAFETY: `self.xa.xa` is always valid by the type invariant.
+ let ptr = unsafe { bindings::xa_load(self.xa.xa.get(), to_index(index)) };
+ let ptr = NonNull::new(ptr.cast())?;
+ Some(f(ptr))
+ }
+
+ /// Loads an entry from the array.
+ ///
+ /// Returns the entry at the given index.
+ pub fn get(&self, index: usize) -> Option<T::Borrowed<'_>> {
+ self.load(index, |ptr| {
+ // SAFETY: `ptr` came from `T::into_foreign`.
+ unsafe { T::borrow(ptr.as_ptr()) }
+ })
+ }
+
+ /// Loads an entry from the array.
+ ///
+ /// Returns the entry at the given index.
+ pub fn get_mut(&mut self, index: usize) -> Option<T::BorrowedMut<'_>> {
+ self.load(index, |ptr| {
+ // SAFETY: `ptr` came from `T::into_foreign`.
+ unsafe { T::borrow_mut(ptr.as_ptr()) }
+ })
+ }
+
+ /// Erases an entry from the array.
+ ///
+ /// Returns the entry which was previously at the given index.
+ pub fn remove(&mut self, index: usize) -> Option<T> {
+ // SAFETY: `self.xa.xa` is always valid by the type invariant.
+ //
+ // SAFETY: The caller holds the lock.
+ let ptr = unsafe { bindings::__xa_erase(self.xa.xa.get(), to_index(index)) }.cast();
+ // SAFETY: `ptr` is either NULL or came from `T::into_foreign`.
+ unsafe { T::try_from_foreign(ptr) }
+ }
+
+ /// Stores an entry in the array.
+ ///
+ /// May drop the lock if needed to allocate memory, and then reacquire it afterwards.
+ ///
+ /// On success, returns the entry which was previously at the given index.
+ ///
+ /// On failure, returns the entry which was attempted to be stored.
+ pub fn store(
+ &mut self,
+ index: usize,
+ value: T,
+ gfp: alloc::Flags,
+ ) -> Result<Option<T>, StoreError<T>> {
+ build_assert!(
+ mem::align_of::<T::PointedTo>() >= 4,
+ "pointers stored in XArray must be 4-byte aligned"
+ );
+ let new = value.into_foreign();
+
+ let old = {
+ let new = new.cast();
+ // SAFETY: `self.xa.xa` is always valid by the type invariant.
+ //
+ // SAFETY: The caller holds the lock.
+ //
+ // INVARIANT: `new` came from `T::into_foreign`.
+ unsafe { bindings::__xa_store(self.xa.xa.get(), to_index(index), new, gfp.as_raw()) }
+ };
+
+ // SAFETY: `__xa_store` returns the old entry at this index on success or `xa_err` if an
+ // error happened.
+ let errno = unsafe { bindings::xa_err(old) };
+ if errno != 0 {
+ // SAFETY: `new` came from `T::into_foreign` and `__xa_store` does not take
+ // ownership of the value on error.
+ let value = unsafe { T::from_foreign(new) };
+ Err(StoreError {
+ value,
+ error: Error::from_errno(errno),
+ })
+ } else {
+ let old = old.cast();
+ // SAFETY: `ptr` is either NULL or came from `T::into_foreign`.
+ //
+ // NB: `XA_ZERO_ENTRY` is never returned by functions belonging to the Normal XArray
+ // API; such entries present as `NULL`.
+ Ok(unsafe { T::try_from_foreign(old) })
+ }
+ }
+}
+
+// SAFETY: `XArray<T>` has no shared mutable state so it is `Send` iff `T` is `Send`.
+unsafe impl<T: ForeignOwnable + Send> Send for XArray<T> {}
+
+// SAFETY: `XArray<T>` serialises the interior mutability it provides so it is `Sync` iff `T` is
+// `Send`.
+unsafe impl<T: ForeignOwnable + Send> Sync for XArray<T> {}

--
2.47.1