[PATCH v4 06/28] rust: alloc: implement `Vmalloc` allocator

[Danilo Krummrich <dakr@xxxxxxxxxx>]

Implement `Allocator` for `Vmalloc`, the kernel's virtually contiguous
allocator, typically used for larger objects, (much) larger than page
size.

All memory allocations made with `Vmalloc` end up in `vrealloc()`.

Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
---
 rust/helpers.c                      |  7 +++++++
 rust/kernel/alloc/allocator.rs      | 32 +++++++++++++++++++++++++++++
 rust/kernel/alloc/allocator_test.rs |  1 +
 3 files changed, 40 insertions(+)

diff --git a/rust/helpers.c b/rust/helpers.c
index 9f7275493365..7406943f887d 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -33,6 +33,7 @@
 #include <linux/sched/signal.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/vmalloc.h>
 #include <linux/wait.h>
 #include <linux/workqueue.h>
 
@@ -199,6 +200,12 @@ void *rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
 }
 EXPORT_SYMBOL_GPL(rust_helper_krealloc);
 
+void *rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
+{
+	return vrealloc(p, size, flags);
+}
+EXPORT_SYMBOL_GPL(rust_helper_vrealloc);
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index c6ad1dd59dd0..bb55895cbd03 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -9,6 +9,7 @@
 
 use crate::alloc::{AllocError, Allocator};
 use crate::bindings;
+use crate::pr_warn;
 
 /// The contiguous kernel allocator.
 ///
@@ -16,6 +17,12 @@
 /// `bindings::krealloc`.
 pub struct Kmalloc;
 
+/// The virtually contiguous kernel allocator.
+///
+/// The vmalloc allocator allocates pages from the page level allocator and maps them into the
+/// contiguous kernel virtual space.
+pub struct Vmalloc;
+
 /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
 fn aligned_size(new_layout: Layout) -> usize {
     // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
@@ -61,6 +68,11 @@ fn krealloc() -> Self {
         Self(bindings::krealloc)
     }
 
+    // INVARIANT: `vrealloc` satisfies the type invariants.
+    fn vrealloc() -> Self {
+        Self(bindings::vrealloc)
+    }
+
     /// # Safety
     ///
     /// This method has the exact same safety requirements as `Allocator::realloc`.
@@ -141,6 +153,26 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
     }
 }
 
+unsafe impl Allocator for Vmalloc {
+    unsafe fn realloc(
+        ptr: Option<NonNull<u8>>,
+        layout: Layout,
+        flags: Flags,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        let realloc = ReallocFunc::vrealloc();
+
+        // TODO: Support alignments larger than PAGE_SIZE.
+        if layout.align() > bindings::PAGE_SIZE {
+            pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
+            return Err(AllocError);
+        }
+
+        // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
+        // allocated with this `Allocator`.
+        unsafe { realloc.call(ptr, layout, flags) }
+    }
+}
+
 #[global_allocator]
 static ALLOCATOR: Kmalloc = Kmalloc;
 
diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
index 4785efc474a7..e7bf2982f68f 100644
--- a/rust/kernel/alloc/allocator_test.rs
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -7,6 +7,7 @@
 use core::ptr::NonNull;
 
 pub struct Kmalloc;
+pub type Vmalloc = Kmalloc;
 
 unsafe impl Allocator for Kmalloc {
     unsafe fn realloc(
-- 
2.45.2