rust: alloc: introduce allocation flags

We'll use them when allocating `Box`, `Arc`, and `UniqueArc` instances,
as well as when allocating memory for `Vec` elements. These changes will
come in subsequent patches.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20240328013603.206764-6-wedsonaf@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc.rs

@@ -6,3 +6,64 @@
 #[cfg(not(testlib))]
 mod allocator;
 pub mod vec_ext;
+
+/// Flags to be used when allocating memory.
+///
+/// They can be combined with the operators `|`, `&`, and `!`.
+///
+/// Values can be used from the [`flags`] module.
+#[derive(Clone, Copy)]
+pub struct Flags(u32);
+
+impl core::ops::BitOr for Flags {
+    type Output = Self;
+    fn bitor(self, rhs: Self) -> Self::Output {
+        Self(self.0 | rhs.0)
+    }
+}
+
+impl core::ops::BitAnd for Flags {
+    type Output = Self;
+    fn bitand(self, rhs: Self) -> Self::Output {
+        Self(self.0 & rhs.0)
+    }
+}
+
+impl core::ops::Not for Flags {
+    type Output = Self;
+    fn not(self) -> Self::Output {
+        Self(!self.0)
+    }
+}
+
+/// Allocation flags.
+///
+/// These are meant to be used in functions that can allocate memory.
+pub mod flags {
+    use super::Flags;
+    use crate::bindings;
+
+    /// Zeroes out the allocated memory.
+    ///
+    /// This is normally or'd with other flags.
+    pub const __GFP_ZERO: Flags = Flags(bindings::__GFP_ZERO);
+
+    /// Users can not sleep and need the allocation to succeed.
+    ///
+    /// A lower watermark is applied to allow access to "atomic reserves". The current
+    /// implementation doesn't support NMI and few other strict non-preemptive contexts (e.g.
+    /// raw_spin_lock). The same applies to [`GFP_NOWAIT`].
+    pub const GFP_ATOMIC: Flags = Flags(bindings::GFP_ATOMIC);
+
+    /// Typical for kernel-internal allocations. The caller requires ZONE_NORMAL or a lower zone
+    /// for direct access but can direct reclaim.
+    pub const GFP_KERNEL: Flags = Flags(bindings::GFP_KERNEL);
+
+    /// The same as [`GFP_KERNEL`], except the allocation is accounted to kmemcg.
+    pub const GFP_KERNEL_ACCOUNT: Flags = Flags(bindings::GFP_KERNEL_ACCOUNT);
+
+    /// Ror kernel allocations that should not stall for direct reclaim, start physical IO or
+    /// use any filesystem callback.  It is very likely to fail to allocate memory, even for very
+    /// small allocations.
+    pub const GFP_NOWAIT: Flags = Flags(bindings::GFP_NOWAIT);
+}