linux-kernel - [PATCH v2 3/7] rust: alloc: add Vec::push_within

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Message-ID: <20250321-vec-methods-v2-3-6d9c8a4634cb@google.com>
Date: Fri, 21 Mar 2025 12:09:58 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Matthew Maurer <mmaurer@...gle.com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Alice Ryhl <aliceryhl@...gle.com>, 
	Benno Lossin <benno.lossin@...ton.me>
Subject: [PATCH v2 3/7] rust: alloc: add Vec::push_within_capacity

This introduces a new method called `push_within_capacity` for appending
to a vector without attempting to allocate if the capacity is full. Rust
Binder will use this in various places to safely push to a vector while
holding a spinlock.

The existing Vec::push method is reimplemented in terms of the new
method.

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 rust/kernel/alloc/kvec.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index e42c5c40c992c796409505da70975f970524fc24..c9de619466eed1590fda039a291207100729a96a 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -284,6 +284,31 @@ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
     /// ```
     pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
         self.reserve(1, flags)?;
+        let err = self.push_within_capacity(v);
+        // SAFETY: The call to `reserve` was successful, so `push_within_capacity` cannot fail.
+        unsafe { err.unwrap_unchecked() };
+        Ok(())
+    }
+
+    /// Appends an element to the back of the [`Vec`] instance.
+    ///
+    /// Fails if the vector does not have capacity for the new element.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = KVec::with_capacity(10, GFP_KERNEL);
+    /// for i in 0..10 {
+    ///     v.push_within_capacity(i).unwrap();
+    /// }
+    ///
+    /// assert!(v.push_within_capacity(10).is_err());
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn push_within_capacity(&mut self, v: T) -> Result<(), T> {
+        if self.len() >= self.capacity() {
+            return Err(v);
+        }
 
         // SAFETY:
         // - `self.len` is smaller than `self.capacity` and hence, the resulting pointer is

-- 
2.49.0.395.g12beb8f557-goog