1// SPDX-License-Identifier: GPL-2.0
2
3//! Allocator support.
4//!
5//! Documentation for the kernel's memory allocators can found in the "Memory Allocation Guide"
6//! linked below. For instance, this includes the concept of "get free page" (GFP) flags and the
7//! typical application of the different kernel allocators.
8//!
9//! Reference: <https://docs.kernel.org/core-api/memory-allocation.html>
10
11use super::Flags;
12use core::alloc::Layout;
13use core::ptr;
14use core::ptr::NonNull;
15
16use crate::alloc::{AllocError, Allocator};
17use crate::bindings;
18use crate::pr_warn;
19
20/// The contiguous kernel allocator.
21///
22/// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also
23/// supports larger allocations up to `bindings::KMALLOC_MAX_SIZE`, which is hardware specific.
24///
25/// For more details see [self].
26pub struct Kmalloc;
27
28/// The virtually contiguous kernel allocator.
29///
30/// `Vmalloc` allocates pages from the page level allocator and maps them into the contiguous kernel
31/// virtual space. It is typically used for large allocations. The memory allocated with this
32/// allocator is not physically contiguous.
33///
34/// For more details see [self].
35pub struct Vmalloc;
36
37/// The kvmalloc kernel allocator.
38///
39/// `KVmalloc` attempts to allocate memory with `Kmalloc` first, but falls back to `Vmalloc` upon
40/// failure. This allocator is typically used when the size for the requested allocation is not
41/// known and may exceed the capabilities of `Kmalloc`.
42///
43/// For more details see [self].
44pub struct KVmalloc;
45
46/// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
47fn aligned_size(new_layout: Layout) -> usize {
48 // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
49 let layout = new_layout.pad_to_align();
50
51 // Note that `layout.size()` (after padding) is guaranteed to be a multiple of `layout.align()`
52 // which together with the slab guarantees means the `krealloc` will return a properly aligned
53 // object (see comments in `kmalloc()` for more information).
54 layout.size()
55}
56
57/// # Invariants
58///
59/// One of the following: `krealloc`, `vrealloc`, `kvrealloc`.
60struct ReallocFunc(
61 unsafe extern "C" fn(*const crate::ffi::c_void, usize, u32) -> *mut crate::ffi::c_void,
62);
63
64impl ReallocFunc {
65 // INVARIANT: `krealloc` satisfies the type invariants.
66 const KREALLOC: Self = Self(bindings::krealloc);
67
68 // INVARIANT: `vrealloc` satisfies the type invariants.
69 const VREALLOC: Self = Self(bindings::vrealloc);
70
71 // INVARIANT: `kvrealloc` satisfies the type invariants.
72 const KVREALLOC: Self = Self(bindings::kvrealloc);
73
74 /// # Safety
75 ///
76 /// This method has the same safety requirements as [`Allocator::realloc`].
77 ///
78 /// # Guarantees
79 ///
80 /// This method has the same guarantees as `Allocator::realloc`. Additionally
81 /// - it accepts any pointer to a valid memory allocation allocated by this function.
82 /// - memory allocated by this function remains valid until it is passed to this function.
83 #[inline]
84 unsafe fn call(
85 &self,
86 ptr: Option<NonNull<u8>>,
87 layout: Layout,
88 old_layout: Layout,
89 flags: Flags,
90 ) -> Result<NonNull<[u8]>, AllocError> {
91 let size = aligned_size(layout);
92 let ptr = match ptr {
93 Some(ptr) => {
94 if old_layout.size() == 0 {
95 ptr::null()
96 } else {
97 ptr.as_ptr()
98 }
99 }
100 None => ptr::null(),
101 };
102
103 // SAFETY:
104 // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc` and thus only requires that
105 // `ptr` is NULL or valid.
106 // - `ptr` is either NULL or valid by the safety requirements of this function.
107 //
108 // GUARANTEE:
109 // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc`.
110 // - Those functions provide the guarantees of this function.
111 let raw_ptr = unsafe {
112 // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
113 self.0(ptr.cast(), size, flags.0).cast()
114 };
115
116 let ptr = if size == 0 {
117 crate::alloc::dangling_from_layout(layout)
118 } else {
119 NonNull::new(raw_ptr).ok_or(AllocError)?
120 };
121
122 Ok(NonNull::slice_from_raw_parts(ptr, size))
123 }
124}
125
126// SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
127// - memory remains valid until it is explicitly freed,
128// - passing a pointer to a valid memory allocation is OK,
129// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
130unsafe impl Allocator for Kmalloc {
131 #[inline]
132 unsafe fn realloc(
133 ptr: Option<NonNull<u8>>,
134 layout: Layout,
135 old_layout: Layout,
136 flags: Flags,
137 ) -> Result<NonNull<[u8]>, AllocError> {
138 // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
139 unsafe { ReallocFunc::KREALLOC.call(ptr, layout, old_layout, flags) }
140 }
141}
142
143// SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
144// - memory remains valid until it is explicitly freed,
145// - passing a pointer to a valid memory allocation is OK,
146// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
147unsafe impl Allocator for Vmalloc {
148 #[inline]
149 unsafe fn realloc(
150 ptr: Option<NonNull<u8>>,
151 layout: Layout,
152 old_layout: Layout,
153 flags: Flags,
154 ) -> Result<NonNull<[u8]>, AllocError> {
155 // TODO: Support alignments larger than PAGE_SIZE.
156 if layout.align() > bindings::PAGE_SIZE {
157 pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
158 return Err(AllocError);
159 }
160
161 // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
162 // allocated with this `Allocator`.
163 unsafe { ReallocFunc::VREALLOC.call(ptr, layout, old_layout, flags) }
164 }
165}
166
167// SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
168// - memory remains valid until it is explicitly freed,
169// - passing a pointer to a valid memory allocation is OK,
170// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
171unsafe impl Allocator for KVmalloc {
172 #[inline]
173 unsafe fn realloc(
174 ptr: Option<NonNull<u8>>,
175 layout: Layout,
176 old_layout: Layout,
177 flags: Flags,
178 ) -> Result<NonNull<[u8]>, AllocError> {
179 // TODO: Support alignments larger than PAGE_SIZE.
180 if layout.align() > bindings::PAGE_SIZE {
181 pr_warn!("KVmalloc does not support alignments larger than PAGE_SIZE yet.\n");
182 return Err(AllocError);
183 }
184
185 // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
186 // allocated with this `Allocator`.
187 unsafe { ReallocFunc::KVREALLOC.call(ptr, layout, old_layout, flags) }
188 }
189}
190

source code of linux/rust/kernel/alloc/allocator.rs

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