Re: [PATCH v5] rust: xarray: Add an abstraction for XArray

[Andreas Hindborg <a.hindborg@samsung.com>]

Hi Maira,

Thanks for working on this!

I have a few comments. I think most have been brought up in earlier
revisions, but since they are not resolved, I put them again below.

Maíra Canal <mcanal@igalia.com> writes:
> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> new file mode 100644
> index 000000000000..1aa77e6abfd4
> --- /dev/null
> +++ b/rust/kernel/xarray.rs
> @@ -0,0 +1,340 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! XArray abstraction.
> +//!
> +//! C header: [`include/linux/xarray.h`](../../include/linux/xarray.h)
> +
> +use crate::{
> +    bindings,
> +    error::{to_result, Error, Result},
> +    types::{ForeignOwnable, Opaque, ScopeGuard},
> +};
> +use core::{
> +    marker::{PhantomData, PhantomPinned},
> +    ops::Deref,
> +    pin::Pin,
> +    ptr::NonNull,
> +};
> +
> +/// Flags passed to `XArray::new` to configure the `XArray`.
> +type Flags = bindings::gfp_t;
> +
> +/// Flag values passed to `XArray::new` to configure the `XArray`.
> +pub mod flags {
> +    /// Use IRQ-safe locking.
> +    pub const LOCK_IRQ: super::Flags = bindings::BINDINGS_XA_FLAGS_LOCK_IRQ;
> +    /// Use softirq-safe locking.
> +    pub const LOCK_BH: super::Flags = bindings::BINDINGS_XA_FLAGS_LOCK_BH;
> +    /// Track which entries are free (distinct from None).
> +    pub const TRACK_FREE: super::Flags = bindings::BINDINGS_XA_FLAGS_TRACK_FREE;
> +    /// Initialize array index 0 as busy.
> +    pub const ZERO_BUSY: super::Flags = bindings::BINDINGS_XA_FLAGS_ZERO_BUSY;
> +    /// Use GFP_ACCOUNT for internal memory allocations.
> +    pub const ACCOUNT: super::Flags = bindings::BINDINGS_XA_FLAGS_ACCOUNT;
> +    /// Create an allocating `XArray` starting at index 0.
> +    pub const ALLOC: super::Flags = bindings::BINDINGS_XA_FLAGS_ALLOC;
> +    /// Create an allocating `XArray` starting at index 1.
> +    pub const ALLOC1: super::Flags = bindings::BINDINGS_XA_FLAGS_ALLOC1;
> +}
> +
> +/// Wrapper for a value owned by the `XArray` which holds the `XArray` lock until dropped.
> +pub struct Guard<'a, T: ForeignOwnable>(NonNull<T>, Pin<&'a XArray<T>>);
> +
> +// INVARIANTS: The lock held by the Guard prevents concurrent access to the XArray.
> +// The Guard's Drop implementation releases the lock only when the Guard is
> +// dropped, ensuring exclusive access during its existence. Therefore, within
> +// the Guard's lifetime, no other operation (including the XArray
> +// calling `T::from_foreign`) can access or modify the underlying value.
> +//
> +// The Guard holds a reference (`self.0`) to the underlying value owned by
> +// XArray. You can use the `into_foreign` method to obtain a pointer to the
> +// foreign representation of the owned value, which is valid for the lifetime
> +// of the Guard.
> +impl<'a, T: ForeignOwnable> Guard<'a, T> {
> +    /// Borrow the underlying value wrapped by the `Guard`.
> +    ///
> +    /// Returns a `T::Borrowed` type for the owned `ForeignOwnable` type.
> +    pub fn borrow(&self) -> T::Borrowed<'_> {
> +        // SAFETY: The value is owned by the `XArray`, the lifetime it is borrowed for must not
> +        // outlive the `XArray` itself, nor the Guard that holds the lock ensuring the value
> +        // remains in the `XArray`.
> +        unsafe { T::borrow(self.0.as_ptr() as _) }

I think specifying the target type of the cast here is preferable. Then
the code will not break silently if something should change.

> +    }
> +}
> +
> +// Convenience impl for `ForeignOwnable` types whose `Borrowed`
> +// form implements Deref.
> +impl<'a, T: ForeignOwnable> Deref for Guard<'a, T>
> +where
> +    T::Borrowed<'a>: Deref,
> +    for<'b> T::Borrowed<'b>: Into<&'b <T::Borrowed<'a> as Deref>::Target>,
> +{
> +    type Target = <T::Borrowed<'a> as Deref>::Target;
> +
> +    fn deref(&self) -> &Self::Target {
> +        self.borrow().into()
> +    }
> +}
> +
> +impl<'a, T: ForeignOwnable> Drop for Guard<'a, T> {
> +    fn drop(&mut self) {
> +        // SAFETY: The XArray we have a reference to owns the C xarray object.
> +        unsafe { bindings::xa_unlock(self.1.xa.get()) };
> +    }
> +}
> +
> +/// Represents a reserved slot in an `XArray`, which does not yet have a value but has an assigned
> +/// index and may not be allocated by any other user. If the Reservation is dropped without
> +/// being filled, the entry is marked as available again.
> +///
> +/// Users must ensure that reserved slots are not filled by other mechanisms, or otherwise their
> +/// contents may be dropped and replaced (which will print a warning).
> +pub struct Reservation<'a, T: ForeignOwnable>(Pin<&'a XArray<T>>, usize, PhantomData<T>);
> +
> +impl<'a, T: ForeignOwnable> Reservation<'a, T> {
> +    /// Stores a value into the reserved slot.
> +    pub fn store(self, value: T) -> Result<usize> {
> +        if self.0.replace(self.1, value)?.is_some() {
> +            crate::pr_err!("XArray: Reservation stored but the entry already had data!\n");
> +            // Consider it a success anyway, not much we can do
> +        }
> +        let index = self.1;
> +        // The reservation is now fulfilled, so do not run our destructor.
> +        core::mem::forget(self);
> +        Ok(index)
> +    }
> +
> +    /// Returns the index of this reservation.
> +    pub fn index(&self) -> usize {
> +        self.1
> +    }
> +}
> +
> +impl<'a, T: ForeignOwnable> Drop for Reservation<'a, T> {
> +    fn drop(&mut self) {
> +        if self.0.remove(self.1).is_some() {
> +            crate::pr_err!("XArray: Reservation dropped but the entry was not empty!\n");
> +        }
> +    }
> +}
> +
> +/// An array which efficiently maps sparse integer indices to owned objects.
> +///
> +/// This is similar to a `Vec<Option<T>>`, but more efficient when there are holes in the
> +/// index space, and can be efficiently grown.
> +///
> +/// This structure is expected to often be used with an inner type that
> +/// can be efficiently cloned, such as an `Arc<T>`.
> +pub struct XArray<T: ForeignOwnable> {
> +    xa: Opaque<bindings::xarray>,
> +    _p: PhantomData<T>,
> +    _q: PhantomPinned,
> +}
> +
> +impl<T: ForeignOwnable> XArray<T> {
> +    /// Creates a new `XArray` with the given flags.
> +    pub fn new(flags: Flags) -> XArray<T> {
> +        let xa = Opaque::uninit();
> +
> +        // SAFETY: We have just created `xa`. This data structure does not require
> +        // pinning.
> +        unsafe { bindings::xa_init_flags(xa.get(), flags) };
> +
> +        // INVARIANT: All pointers stored in the array are pointers obtained by
> +        // calling `T::into_foreign`.
> +        XArray {
> +            xa,
> +            _p: PhantomData,
> +            _q: PhantomPinned,
> +        }
> +    }

Since `xarray` holds a lock, I think it should be pinned. I think we
discussed that for x86 it does not need to be pinned when it is not
locked, but we are not sure if that holds for all architectures.

Also ergonomics might be better if the constructor returns an
initialization rather than the caller having to pin after initialization.

> +
> +    /// Replaces an entry with a new value, returning the old value (if any).
> +    pub fn replace(self: Pin<&Self>, index: usize, value: T) -> Result<Option<T>> {
> +        let new = value.into_foreign();
> +        // SAFETY: `new` just came from into_foreign(), and we dismiss this guard if
> +        // the xa_store operation succeeds and takes ownership of the pointer.
> +        let guard = ScopeGuard::new(|| unsafe {
> +            T::from_foreign(new);
> +        });
> +
> +        // SAFETY: `self.xa` is always valid by the type invariant, and we are storing
> +        // a `T::into_foreign()` result which upholds the later invariants.

T has to be 4-byte aligned according to xarray docs. All types that we
implement `ForeignOwnable` for is aligned such, because
`KernelAllocator` is producing allocations of alignment
`ARCH_SLAB_MINALIGN`, which is `__alignof__(unsigned long long)`. But it
should probably be in a safety requirement somewhere. Perhaps on
`ForeignOwnable` and then make that an unsafe trait?

> +        let old = unsafe {
> +            bindings::xa_store(
> +                self.xa.get(),
> +                index.try_into()?,
> +                new as *mut _,

Same here, I would prefer full type.

> +                bindings::GFP_KERNEL,
> +            )
> +        };
> +
> +        // SAFETY: `xa_store` returns the old entry at this index on success or
> +        // a XArray result, which can be turn into an errno through `xa_err`.
> +        to_result(unsafe { bindings::xa_err(old) })?;
> +        guard.dismiss();
> +
> +        Ok(if old.is_null() {
> +            None
> +        } else {
> +            // SAFETY: The old value must have been stored by either this function or
> +            // `alloc_limits_opt`, both of which ensure non-NULL entries are valid
> +            // ForeignOwnable pointers.
> +            Some(unsafe { T::from_foreign(old) })
> +        })
> +    }
> +
> +    /// Replaces an entry with a new value, dropping the old value (if any).
> +    pub fn set(self: Pin<&Self>, index: usize, value: T) -> Result {
> +        self.replace(index, value)?;
> +        Ok(())
> +    }
> +
> +    /// Looks up and returns a reference to an entry in the array, returning a `Guard` if it
> +    /// exists.
> +    ///
> +    /// This guard blocks all other actions on the `XArray`. Callers are expected to drop the
> +    /// `Guard` eagerly to avoid blocking other users, such as by taking a clone of the value.
> +    pub fn get(self: Pin<&Self>, index: usize) -> Option<Guard<'_, T>> {
> +        // SAFETY: `self.xa` is always valid by the type invariant.
> +        unsafe { bindings::xa_lock(self.xa.get()) };
> +
> +        // SAFETY: `self.xa` is always valid by the type invariant.
> +        let guard = ScopeGuard::new(|| unsafe { bindings::xa_unlock(self.xa.get()) });
> +
> +        // SAFETY: `self.xa` is always valid by the type invariant.
> +        let p = unsafe { bindings::xa_load(self.xa.get(), index.try_into().ok()?) };
> +
> +        NonNull::new(p as *mut T).map(|p| {
> +            guard.dismiss();
> +            Guard(p, self)
> +        })
> +    }
> +
> +    /// Removes and returns an entry, returning it if it existed.
> +    pub fn remove(self: Pin<&Self>, index: usize) -> Option<T> {
> +        // SAFETY: `self.xa` is always valid by the type invariant.
> +        let p = unsafe { bindings::xa_erase(self.xa.get(), index.try_into().ok()?) };
> +        if p.is_null() {
> +            None
> +        } else {
> +            // SAFETY: All pointers stored in the array are pointers obtained by
> +            // calling `T::into_foreign`.
> +            Some(unsafe { T::from_foreign(p) })
> +        }
> +    }
> +
> +    /// Allocates a new index in the array, optionally storing a new value into it, with
> +    /// configurable bounds for the index range to allocate from.
> +    ///
> +    /// If `value` is `None`, then the index is reserved from further allocation but remains
> +    /// free for storing a value into it.
> +    fn alloc_limits_opt(self: Pin<&Self>, value: Option<T>, min: u32, max: u32) -> Result<usize> {
> +        let new = value.map_or(core::ptr::null(), |a| a.into_foreign());
> +        let mut id: u32 = 0;
> +
> +        let guard = ScopeGuard::new(|| {
> +            if !new.is_null() {
> +                // SAFETY: If `new` is not NULL, it came from the `ForeignOwnable` we got
> +                // from the caller.
> +                unsafe { T::from_foreign(new) };
> +            }
> +        });
> +
> +        // SAFETY: `self.xa` is always valid by the type invariant. If this succeeds, it
> +        // takes ownership of the passed `T` (if any). If it fails, we must drop the
> +        // `T` again.
> +        let ret = unsafe {
> +            bindings::xa_alloc(
> +                self.xa.get(),
> +                &mut id,
> +                new as *mut _,

Again, I prefer the full type here.

> +                bindings::xa_limit { min, max },
> +                bindings::GFP_KERNEL,
> +            )
> +        };
> +
> +        if ret < 0 {
> +            Err(Error::from_errno(ret))
> +        } else {
> +            guard.dismiss();
> +            Ok(id as usize)
> +        }
> +    }
> +
> +    /// Allocates a new index in the array, storing a new value into it, with configurable
> +    /// bounds for the index range to allocate from.
> +    pub fn alloc_limits(self: Pin<&Self>, value: T, min: u32, max: u32) -> Result<usize> {
> +        self.alloc_limits_opt(Some(value), min, max)
> +    }
> +
> +    /// Allocates a new index in the array, storing a new value into it.
> +    pub fn alloc(self: Pin<&Self>, value: T) -> Result<usize> {
> +        self.alloc_limits(value, 0, u32::MAX)
> +    }
> +
> +    /// Reserves a new index in the array within configurable bounds for the index.
> +    ///
> +    /// Returns a `Reservation` object, which can then be used to store a value at this index or
> +    /// otherwise free it for reuse.
> +    pub fn reserve_limits(self: Pin<&Self>, min: u32, max: u32) -> Result<Reservation<'_, T>> {
> +        Ok(Reservation(
> +            self,
> +            self.alloc_limits_opt(None, min, max)?,
> +            PhantomData,
> +        ))
> +    }
> +
> +    /// Reserves a new index in the array.
> +    ///
> +    /// Returns a `Reservation` object, which can then be used to store a value at this index or
> +    /// otherwise free it for reuse.
> +    pub fn reserve(self: Pin<&Self>) -> Result<Reservation<'_, T>> {
> +        Ok(Reservation(
> +            self,
> +            self.alloc_limits_opt(None, 0, u32::MAX)?,
> +            PhantomData,
> +        ))
> +    }
> +}
> +
> +impl<T: ForeignOwnable> Drop for XArray<T> {
> +    fn drop(&mut self) {
> +        let mut index: core::ffi::c_ulong = 0;
> +
> +        // SAFETY: `self.xa` is valid by the type invariant, and as we have
> +        // the only reference to the `XArray` we can safely iterate its contents
> +        // and drop everything.
> +        unsafe {
> +            let mut entry = bindings::xa_find(
> +                self.xa.get(),
> +                &mut index,
> +                core::ffi::c_ulong::MAX,
> +                bindings::BINDINGS_XA_PRESENT,
> +            );
> +
> +            while !entry.is_null() {
> +                T::from_foreign(entry);
> +                entry = bindings::xa_find_after(
> +                    self.xa.get(),
> +                    &mut index,
> +                    core::ffi::c_ulong::MAX,
> +                    bindings::BINDINGS_XA_PRESENT,
> +                );
> +            }
> +        }
> +
> +        // SAFETY: Locked locks are not safe to drop. Normally we would want to
> +        // try_lock()/unlock() here for safety or something similar, but in this
> +        // case xa_destroy() is guaranteed to acquire the lock anyway. This will
> +        // deadlock if a lock guard was improperly dropped, but that is not UB,
> +        // so it's sufficient for soundness purposes.
> +        unsafe {
> +            bindings::xa_destroy(self.xa.get());
> +        }
> +    }
> +}
> +
> +// SAFETY: XArray is thread-safe and all mutation operations are internally locked.
> +unsafe impl<T: Send + ForeignOwnable> Send for XArray<T> {}
> +unsafe impl<T: Sync + ForeignOwnable> Sync for XArray<T> {}

Do we actually need to specify `ForeignOwnable` here?


Best regards,
Andreas Hindborg