1 use alloc::boxed::Box; 2 3 use crate::*; 4 use core::{ 5 mem, 6 sync::atomic::{AtomicU64, Ordering}, 7 }; 8 9 /// A trait defining bitfield operations we need for tracking allocated objects within a page. 10 pub(crate) trait Bitfield { 11 fn initialize(&mut self, for_size: usize, capacity: usize); 12 fn first_fit( 13 &self, 14 base_addr: usize, 15 layout: Layout, 16 page_size: usize, 17 ) -> Option<(usize, usize)>; 18 fn is_allocated(&self, idx: usize) -> bool; 19 fn set_bit(&self, idx: usize); 20 fn clear_bit(&self, idx: usize); 21 fn is_full(&self) -> bool; 22 fn all_free(&self, relevant_bits: usize) -> bool; 23 } 24 25 /// Implementation of bit operations on u64 slices. 26 /// 27 /// We allow deallocations (i.e. clearning a bit in the field) 28 /// from any thread. That's why the bitfield is a bunch of AtomicU64. 29 impl Bitfield for [AtomicU64] { 30 /// Initialize the bitfield 31 /// 32 /// # Arguments 33 /// * `for_size`: Object size we want to allocate 34 /// * `capacity`: Maximum size of the buffer the bitmap maintains. 35 /// 36 /// Ensures that we only have free slots for what we can allocate 37 /// within the page (by marking everything else allocated). 38 fn initialize(&mut self, for_size: usize, capacity: usize) { 39 // Set everything to allocated 40 for bitmap in self.iter_mut() { 41 *bitmap = AtomicU64::new(u64::max_value()); 42 } 43 44 // Mark actual slots as free 45 let relevant_bits = core::cmp::min(capacity / for_size, self.len() * 64); 46 for idx in 0..relevant_bits { 47 self.clear_bit(idx); 48 } 49 } 50 51 /// Tries to find a free block of memory that satisfies `alignment` requirement. 52 /// 53 /// # Notes 54 /// * We pass size here to be able to calculate the resulting address within `data`. 55 #[inline(always)] 56 fn first_fit( 57 &self, 58 base_addr: usize, 59 layout: Layout, 60 page_size: usize, 61 ) -> Option<(usize, usize)> { 62 let start_offset = get_offset_for_align(layout); 63 let data_start = base_addr + start_offset; 64 65 for (base_idx, b) in self.iter().enumerate() { 66 let bitval = b.load(Ordering::Relaxed); 67 if bitval == u64::max_value() { 68 continue; 69 } else { 70 let negated = !bitval; 71 let first_free = negated.trailing_zeros() as usize; 72 let idx: usize = base_idx * 64 + first_free; 73 let offset = idx * layout.size(); 74 75 // TODO(bad): psize needs to be passed as arg 76 let offset_inside_data_area = 77 offset <= (page_size - OBJECT_PAGE_METADATA_OVERHEAD - layout.size()); 78 if !offset_inside_data_area { 79 return None; 80 } 81 82 let addr: usize = data_start + offset; 83 let alignment_ok = addr % layout.align() == 0; 84 let block_is_free = bitval & (1 << first_free) == 0; 85 if alignment_ok && block_is_free { 86 return Some((idx, addr)); 87 } 88 } 89 } 90 None 91 } 92 93 /// Check if the bit `idx` is set. 94 #[inline(always)] 95 fn is_allocated(&self, idx: usize) -> bool { 96 let base_idx = idx / 64; 97 let bit_idx = idx % 64; 98 (self[base_idx].load(Ordering::Relaxed) & (1 << bit_idx)) > 0 99 } 100 101 /// Sets the bit number `idx` in the bit-field. 102 #[inline(always)] 103 fn set_bit(&self, idx: usize) { 104 let base_idx = idx / 64; 105 let bit_idx = idx % 64; 106 self[base_idx].fetch_or(1 << bit_idx, Ordering::Relaxed); 107 } 108 109 /// Clears bit number `idx` in the bit-field. 110 #[inline(always)] 111 fn clear_bit(&self, idx: usize) { 112 let base_idx = idx / 64; 113 let bit_idx = idx % 64; 114 self[base_idx].fetch_and(!(1 << bit_idx), Ordering::Relaxed); 115 } 116 117 /// Checks if we could allocate more objects of a given `alloc_size` within the 118 /// `capacity` of the memory allocator. 119 /// 120 /// # Note 121 /// The ObjectPage will make sure to mark the top-most bits as allocated 122 /// for large sizes (i.e., a size 512 SCAllocator will only really need 3 bits) 123 /// to track allocated objects). That's why this function can be simpler 124 /// than it would need to be in practice. 125 #[inline(always)] 126 fn is_full(&self) -> bool { 127 self.iter() 128 .filter(|&x| x.load(Ordering::Relaxed) != u64::max_value()) 129 .count() 130 == 0 131 } 132 133 /// Checks if the page has currently no allocations. 134 /// 135 /// This is called `all_free` rather than `is_emtpy` because 136 /// we already have an is_empty fn as part of the slice. 137 #[inline(always)] 138 fn all_free(&self, relevant_bits: usize) -> bool { 139 for (idx, bitmap) in self.iter().enumerate() { 140 let checking_bit_range = (idx * 64, (idx + 1) * 64); 141 if relevant_bits >= checking_bit_range.0 && relevant_bits < checking_bit_range.1 { 142 // Last relevant bitmap, here we only have to check that a subset of bitmap is marked free 143 // the rest will be marked full 144 let bits_that_should_be_free = relevant_bits - checking_bit_range.0; 145 let free_mask = (1 << bits_that_should_be_free) - 1; 146 return (free_mask & bitmap.load(Ordering::Relaxed)) == 0; 147 } 148 149 if bitmap.load(Ordering::Relaxed) == 0 { 150 continue; 151 } else { 152 return false; 153 } 154 } 155 156 true 157 } 158 } 159 160 /// # get_offset_for_align - 根据布局大小获取page内对齐偏移量 161 /// 162 /// 这个函数根据给定的`Layout`大小确定一个合适的对齐偏移量。 163 /// 164 /// ## 参数 165 /// 166 /// - layout: Layout，这是需要计算对齐偏移量的布局参数。 167 /// 168 /// ## 返回值 169 /// 170 /// - usize: 成功时返回一个usize类型的对齐偏移量。 171 fn get_offset_for_align(layout: Layout) -> usize { 172 match layout.size() { 173 0..=8 => 80, 174 9..=16 => 80, 175 17..=32 => 96, 176 33..=64 => 128, 177 65..=128 => 128, 178 129..=256 => 256, 179 257..=512 => 512, 180 513..=1024 => 1024, 181 1025..=2048 => 2048, 182 _ => panic!(), 183 } 184 } 185 186 /// This trait is used to define a page from which objects are allocated 187 /// in an `SCAllocator`. 188 /// 189 /// The implementor of this trait needs to provide access to the page meta-data, 190 /// which consists of: 191 /// - A bitfield (to track allocations), 192 /// - `prev` and `next` pointers to insert the page in free lists 193 pub trait AllocablePage { 194 /// The total size (in bytes) of the page. 195 /// 196 /// # Note 197 /// We also assume that the address of the page will be aligned to `SIZE`. 198 const SIZE: usize; 199 200 fn bitfield(&self) -> &[AtomicU64; 8]; 201 fn bitfield_mut(&mut self) -> &mut [AtomicU64; 8]; 202 fn prev(&mut self) -> &mut Rawlink<Self> 203 where 204 Self: core::marker::Sized; 205 fn next(&mut self) -> &mut Rawlink<Self> 206 where 207 Self: core::marker::Sized; 208 209 /// Tries to find a free block within `data` that satisfies `alignment` requirement. 210 fn first_fit(&self, layout: Layout) -> Option<(usize, usize)> { 211 let base_addr = (self as *const Self as *const u8) as usize; 212 self.bitfield().first_fit(base_addr, layout, Self::SIZE) 213 } 214 215 /// Tries to allocate an object within this page. 216 /// 217 /// In case the slab is full, returns a null ptr. 218 fn allocate(&mut self, layout: Layout) -> *mut u8 { 219 match self.first_fit(layout) { 220 Some((idx, addr)) => { 221 self.bitfield().set_bit(idx); 222 addr as *mut u8 223 } 224 None => ptr::null_mut(), 225 } 226 } 227 228 /// Checks if we can still allocate more objects of a given layout within the page. 229 fn is_full(&self) -> bool { 230 self.bitfield().is_full() 231 } 232 233 /// Checks if the page has currently no allocations. 234 fn is_empty(&self, relevant_bits: usize) -> bool { 235 self.bitfield().all_free(relevant_bits) 236 } 237 238 /// Deallocates a memory object within this page. 239 fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) -> Result<(), AllocationError> { 240 trace!( 241 "AllocablePage deallocating ptr = {:p} with {:?}", 242 ptr, 243 layout 244 ); 245 let align_offset = get_offset_for_align(layout); 246 let page_offset = ((ptr.as_ptr() as usize) - align_offset) & (Self::SIZE - 1); 247 assert!(page_offset % layout.size() == 0); 248 let idx = page_offset / layout.size(); 249 assert!( 250 self.bitfield().is_allocated(idx), 251 "{:p} not marked allocated?", 252 ptr 253 ); 254 255 self.bitfield().clear_bit(idx); 256 Ok(()) 257 } 258 } 259 260 /// Holds allocated data within a 4 KiB page. 261 /// 262 /// Has a data-section where objects are allocated from 263 /// and a small amount of meta-data in form of a bitmap 264 /// to track allocations at the end of the page. 265 /// 266 /// # Notes 267 /// An object of this type will be exactly 4 KiB. 268 /// It is marked `repr(C)` because we rely on a well defined order of struct 269 /// members (e.g., dealloc does a cast to find the bitfield). 270 #[repr(C)] 271 pub struct ObjectPage<'a> { 272 #[allow(dead_code)] 273 /// A bit-field to track free/allocated memory within `data`. 274 pub(crate) bitfield: [AtomicU64; 8], 275 276 /// Next element in list (used by `PageList`). 277 next: Rawlink<ObjectPage<'a>>, 278 /// Previous element in list (used by `PageList`) 279 prev: Rawlink<ObjectPage<'a>>, 280 281 /// Holds memory objects. 282 data: [u8; OBJECT_PAGE_SIZE - OBJECT_PAGE_METADATA_OVERHEAD], 283 } 284 285 impl<'a> ObjectPage<'a> { 286 pub fn new() -> Box<ObjectPage<'a>> { 287 unsafe { Box::new_uninit().assume_init() } 288 } 289 } 290 291 // These needs some more work to be really safe... 292 unsafe impl<'a> Send for ObjectPage<'a> {} 293 unsafe impl<'a> Sync for ObjectPage<'a> {} 294 295 impl<'a> AllocablePage for ObjectPage<'a> { 296 const SIZE: usize = OBJECT_PAGE_SIZE; 297 298 fn bitfield(&self) -> &[AtomicU64; 8] { 299 &self.bitfield 300 } 301 fn bitfield_mut(&mut self) -> &mut [AtomicU64; 8] { 302 &mut self.bitfield 303 } 304 305 fn prev(&mut self) -> &mut Rawlink<Self> { 306 &mut self.prev 307 } 308 309 fn next(&mut self) -> &mut Rawlink<Self> { 310 &mut self.next 311 } 312 } 313 314 impl<'a> Default for ObjectPage<'a> { 315 fn default() -> ObjectPage<'a> { 316 unsafe { mem::MaybeUninit::zeroed().assume_init() } 317 } 318 } 319 320 impl<'a> fmt::Debug for ObjectPage<'a> { 321 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { 322 write!(f, "ObjectPage") 323 } 324 } 325 326 /// A list of pages. 327 pub(crate) struct PageList<'a, T: AllocablePage> { 328 /// Points to the head of the list. 329 pub(crate) head: Option<&'a mut T>, 330 /// Number of elements in the list. 331 pub(crate) elements: usize, 332 } 333 334 impl<'a, T: AllocablePage> PageList<'a, T> { 335 #[cfg(feature = "unstable")] 336 pub(crate) const fn new() -> PageList<'a, T> { 337 PageList { 338 head: None, 339 elements: 0, 340 } 341 } 342 343 #[cfg(not(feature = "unstable"))] 344 pub(crate) fn new() -> PageList<'a, T> { 345 PageList { 346 head: None, 347 elements: 0, 348 } 349 } 350 351 pub(crate) fn iter_mut<'b: 'a>(&mut self) -> ObjectPageIterMut<'b, T> { 352 let m = match self.head { 353 None => Rawlink::none(), 354 Some(ref mut m) => Rawlink::some(*m), 355 }; 356 357 ObjectPageIterMut { 358 head: m, 359 phantom: core::marker::PhantomData, 360 } 361 } 362 363 /// Inserts `new_head` at the front of the list. 364 pub(crate) fn insert_front<'b>(&'b mut self, mut new_head: &'a mut T) { 365 match self.head { 366 None => { 367 *new_head.prev() = Rawlink::none(); 368 self.head = Some(new_head); 369 } 370 Some(ref mut head) => { 371 *new_head.prev() = Rawlink::none(); 372 *head.prev() = Rawlink::some(new_head); 373 mem::swap(head, &mut new_head); 374 *head.next() = Rawlink::some(new_head); 375 } 376 } 377 378 self.elements += 1; 379 } 380 381 /// Removes `slab_page` from the list. 382 pub(crate) fn remove_from_list(&mut self, slab_page: &mut T) { 383 unsafe { 384 match slab_page.prev().resolve_mut() { 385 None => { 386 self.head = slab_page.next().resolve_mut(); 387 } 388 Some(prev) => { 389 *prev.next() = match slab_page.next().resolve_mut() { 390 None => Rawlink::none(), 391 Some(next) => Rawlink::some(next), 392 }; 393 } 394 } 395 396 match slab_page.next().resolve_mut() { 397 None => (), 398 Some(next) => { 399 *next.prev() = match slab_page.prev().resolve_mut() { 400 None => Rawlink::none(), 401 Some(prev) => Rawlink::some(prev), 402 }; 403 } 404 } 405 } 406 407 *slab_page.prev() = Rawlink::none(); 408 *slab_page.next() = Rawlink::none(); 409 self.elements -= 1; 410 } 411 412 /// Removes `slab_page` from the list. 413 pub(crate) fn pop<'b>(&'b mut self) -> Option<&'a mut T> { 414 match self.head { 415 None => None, 416 Some(ref mut head) => { 417 let head_next = head.next(); 418 let mut new_head = unsafe { head_next.resolve_mut() }; 419 mem::swap(&mut self.head, &mut new_head); 420 let _ = self.head.as_mut().map(|n| { 421 *n.prev() = Rawlink::none(); 422 }); 423 424 self.elements -= 1; 425 new_head.map(|node| { 426 *node.prev() = Rawlink::none(); 427 *node.next() = Rawlink::none(); 428 node 429 }) 430 } 431 } 432 } 433 434 /// Does the list contain `s`? 435 pub(crate) fn contains(&mut self, s: *const T) -> bool { 436 for slab_page in self.iter_mut() { 437 if core::ptr::eq(slab_page, s) { 438 return true; 439 } 440 } 441 442 false 443 } 444 } 445 446 /// Iterate over all the pages inside a slab allocator 447 pub(crate) struct ObjectPageIterMut<'a, P: AllocablePage> { 448 head: Rawlink<P>, 449 phantom: core::marker::PhantomData<&'a P>, 450 } 451 452 impl<'a, P: AllocablePage + 'a> Iterator for ObjectPageIterMut<'a, P> { 453 type Item = &'a mut P; 454 455 #[inline] 456 fn next(&mut self) -> Option<&'a mut P> { 457 unsafe { 458 self.head.resolve_mut().map(|next| { 459 self.head = match next.next().resolve_mut() { 460 None => Rawlink::none(), 461 Some(ref mut sp) => Rawlink::some(*sp), 462 }; 463 next 464 }) 465 } 466 } 467 } 468 469 /// Rawlink is a type like Option<T> but for holding a raw pointer. 470 /// 471 /// We use it to link AllocablePages together. You probably won't need 472 /// to use this type if you're not implementing AllocablePage 473 /// for a custom page-size. 474 pub struct Rawlink<T> { 475 p: *mut T, 476 } 477 478 impl<T> Default for Rawlink<T> { 479 fn default() -> Self { 480 Rawlink { p: ptr::null_mut() } 481 } 482 } 483 484 impl<T> Rawlink<T> { 485 /// Like Option::None for Rawlink 486 pub(crate) fn none() -> Rawlink<T> { 487 Rawlink { p: ptr::null_mut() } 488 } 489 490 /// Like Option::Some for Rawlink 491 pub(crate) fn some(n: &mut T) -> Rawlink<T> { 492 Rawlink { p: n } 493 } 494 495 /// Convert the `Rawlink` into an Option value 496 /// 497 /// **unsafe** because: 498 /// 499 /// - Dereference of raw pointer. 500 /// - Returns reference of arbitrary lifetime. 501 #[allow(dead_code)] 502 pub(crate) unsafe fn resolve<'a>(&self) -> Option<&'a T> { 503 self.p.as_ref() 504 } 505 506 /// Convert the `Rawlink` into an Option value 507 /// 508 /// **unsafe** because: 509 /// 510 /// - Dereference of raw pointer. 511 /// - Returns reference of arbitrary lifetime. 512 pub(crate) unsafe fn resolve_mut<'a>(&mut self) -> Option<&'a mut T> { 513 self.p.as_mut() 514 } 515 516 /// Return the `Rawlink` and replace with `Rawlink::none()` 517 #[allow(dead_code)] 518 pub(crate) fn take(&mut self) -> Rawlink<T> { 519 mem::replace(self, Rawlink::none()) 520 } 521 } 522