1 // Copyright 2019 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // Page allocator. 6 // 7 // The page allocator manages mapped pages (defined by pageSize, NOT 8 // physPageSize) for allocation and re-use. It is embedded into mheap. 9 // 10 // Pages are managed using a bitmap that is sharded into chunks. 11 // In the bitmap, 1 means in-use, and 0 means free. The bitmap spans the 12 // process's address space. Chunks are managed in a sparse-array-style structure 13 // similar to mheap.arenas, since the bitmap may be large on some systems. 14 // 15 // The bitmap is efficiently searched by using a radix tree in combination 16 // with fast bit-wise intrinsics. Allocation is performed using an address-ordered 17 // first-fit approach. 18 // 19 // Each entry in the radix tree is a summary that describes three properties of 20 // a particular region of the address space: the number of contiguous free pages 21 // at the start and end of the region it represents, and the maximum number of 22 // contiguous free pages found anywhere in that region. 23 // 24 // Each level of the radix tree is stored as one contiguous array, which represents 25 // a different granularity of subdivision of the processes' address space. Thus, this 26 // radix tree is actually implicit in these large arrays, as opposed to having explicit 27 // dynamically-allocated pointer-based node structures. Naturally, these arrays may be 28 // quite large for system with large address spaces, so in these cases they are mapped 29 // into memory as needed. The leaf summaries of the tree correspond to a bitmap chunk. 30 // 31 // The root level (referred to as L0 and index 0 in pageAlloc.summary) has each 32 // summary represent the largest section of address space (16 GiB on 64-bit systems), 33 // with each subsequent level representing successively smaller subsections until we 34 // reach the finest granularity at the leaves, a chunk. 35 // 36 // More specifically, each summary in each level (except for leaf summaries) 37 // represents some number of entries in the following level. For example, each 38 // summary in the root level may represent a 16 GiB region of address space, 39 // and in the next level there could be 8 corresponding entries which represent 2 40 // GiB subsections of that 16 GiB region, each of which could correspond to 8 41 // entries in the next level which each represent 256 MiB regions, and so on. 42 // 43 // Thus, this design only scales to heaps so large, but can always be extended to 44 // larger heaps by simply adding levels to the radix tree, which mostly costs 45 // additional virtual address space. The choice of managing large arrays also means 46 // that a large amount of virtual address space may be reserved by the runtime. 47 48 package runtime 49 50 import ( 51 "runtime/internal/atomic" 52 "unsafe" 53 ) 54 55 const ( 56 // The size of a bitmap chunk, i.e. the amount of bits (that is, pages) to consider 57 // in the bitmap at once. 58 pallocChunkPages = 1 << logPallocChunkPages 59 pallocChunkBytes = pallocChunkPages * pageSize 60 logPallocChunkPages = 9 61 logPallocChunkBytes = logPallocChunkPages + pageShift 62 63 // The number of radix bits for each level. 64 // 65 // The value of 3 is chosen such that the block of summaries we need to scan at 66 // each level fits in 64 bytes (2^3 summaries * 8 bytes per summary), which is 67 // close to the L1 cache line width on many systems. Also, a value of 3 fits 4 tree 68 // levels perfectly into the 21-bit pallocBits summary field at the root level. 69 // 70 // The following equation explains how each of the constants relate: 71 // summaryL0Bits + (summaryLevels-1)*summaryLevelBits + logPallocChunkBytes = heapAddrBits 72 // 73 // summaryLevels is an architecture-dependent value defined in mpagealloc_*.go. 74 summaryLevelBits = 3 75 summaryL0Bits = heapAddrBits - logPallocChunkBytes - (summaryLevels-1)*summaryLevelBits 76 77 // pallocChunksL2Bits is the number of bits of the chunk index number 78 // covered by the second level of the chunks map. 79 // 80 // See (*pageAlloc).chunks for more details. Update the documentation 81 // there should this change. 82 pallocChunksL2Bits = heapAddrBits - logPallocChunkBytes - pallocChunksL1Bits 83 pallocChunksL1Shift = pallocChunksL2Bits 84 ) 85 86 // maxSearchAddr returns the maximum searchAddr value, which indicates 87 // that the heap has no free space. 88 // 89 // This function exists just to make it clear that this is the maximum address 90 // for the page allocator's search space. See maxOffAddr for details. 91 // 92 // It's a function (rather than a variable) because it needs to be 93 // usable before package runtime's dynamic initialization is complete. 94 // See #51913 for details. 95 func maxSearchAddr() offAddr { return maxOffAddr } 96 97 // Global chunk index. 98 // 99 // Represents an index into the leaf level of the radix tree. 100 // Similar to arenaIndex, except instead of arenas, it divides the address 101 // space into chunks. 102 type chunkIdx uint 103 104 // chunkIndex returns the global index of the palloc chunk containing the 105 // pointer p. 106 func chunkIndex(p uintptr) chunkIdx { 107 return chunkIdx((p - arenaBaseOffset) / pallocChunkBytes) 108 } 109 110 // chunkIndex returns the base address of the palloc chunk at index ci. 111 func chunkBase(ci chunkIdx) uintptr { 112 return uintptr(ci)*pallocChunkBytes + arenaBaseOffset 113 } 114 115 // chunkPageIndex computes the index of the page that contains p, 116 // relative to the chunk which contains p. 117 func chunkPageIndex(p uintptr) uint { 118 return uint(p % pallocChunkBytes / pageSize) 119 } 120 121 // l1 returns the index into the first level of (*pageAlloc).chunks. 122 func (i chunkIdx) l1() uint { 123 if pallocChunksL1Bits == 0 { 124 // Let the compiler optimize this away if there's no 125 // L1 map. 126 return 0 127 } else { 128 return uint(i) >> pallocChunksL1Shift 129 } 130 } 131 132 // l2 returns the index into the second level of (*pageAlloc).chunks. 133 func (i chunkIdx) l2() uint { 134 if pallocChunksL1Bits == 0 { 135 return uint(i) 136 } else { 137 return uint(i) & (1<<pallocChunksL2Bits - 1) 138 } 139 } 140 141 // offAddrToLevelIndex converts an address in the offset address space 142 // to the index into summary[level] containing addr. 143 func offAddrToLevelIndex(level int, addr offAddr) int { 144 return int((addr.a - arenaBaseOffset) >> levelShift[level]) 145 } 146 147 // levelIndexToOffAddr converts an index into summary[level] into 148 // the corresponding address in the offset address space. 149 func levelIndexToOffAddr(level, idx int) offAddr { 150 return offAddr{(uintptr(idx) << levelShift[level]) + arenaBaseOffset} 151 } 152 153 // addrsToSummaryRange converts base and limit pointers into a range 154 // of entries for the given summary level. 155 // 156 // The returned range is inclusive on the lower bound and exclusive on 157 // the upper bound. 158 func addrsToSummaryRange(level int, base, limit uintptr) (lo int, hi int) { 159 // This is slightly more nuanced than just a shift for the exclusive 160 // upper-bound. Note that the exclusive upper bound may be within a 161 // summary at this level, meaning if we just do the obvious computation 162 // hi will end up being an inclusive upper bound. Unfortunately, just 163 // adding 1 to that is too broad since we might be on the very edge 164 // of a summary's max page count boundary for this level 165 // (1 << levelLogPages[level]). So, make limit an inclusive upper bound 166 // then shift, then add 1, so we get an exclusive upper bound at the end. 167 lo = int((base - arenaBaseOffset) >> levelShift[level]) 168 hi = int(((limit-1)-arenaBaseOffset)>>levelShift[level]) + 1 169 return 170 } 171 172 // blockAlignSummaryRange aligns indices into the given level to that 173 // level's block width (1 << levelBits[level]). It assumes lo is inclusive 174 // and hi is exclusive, and so aligns them down and up respectively. 175 func blockAlignSummaryRange(level int, lo, hi int) (int, int) { 176 e := uintptr(1) << levelBits[level] 177 return int(alignDown(uintptr(lo), e)), int(alignUp(uintptr(hi), e)) 178 } 179 180 type pageAlloc struct { 181 // Radix tree of summaries. 182 // 183 // Each slice's cap represents the whole memory reservation. 184 // Each slice's len reflects the allocator's maximum known 185 // mapped heap address for that level. 186 // 187 // The backing store of each summary level is reserved in init 188 // and may or may not be committed in grow (small address spaces 189 // may commit all the memory in init). 190 // 191 // The purpose of keeping len <= cap is to enforce bounds checks 192 // on the top end of the slice so that instead of an unknown 193 // runtime segmentation fault, we get a much friendlier out-of-bounds 194 // error. 195 // 196 // To iterate over a summary level, use inUse to determine which ranges 197 // are currently available. Otherwise one might try to access 198 // memory which is only Reserved which may result in a hard fault. 199 // 200 // We may still get segmentation faults < len since some of that 201 // memory may not be committed yet. 202 summary [summaryLevels][]pallocSum 203 204 // chunks is a slice of bitmap chunks. 205 // 206 // The total size of chunks is quite large on most 64-bit platforms 207 // (O(GiB) or more) if flattened, so rather than making one large mapping 208 // (which has problems on some platforms, even when PROT_NONE) we use a 209 // two-level sparse array approach similar to the arena index in mheap. 210 // 211 // To find the chunk containing a memory address `a`, do: 212 // chunkOf(chunkIndex(a)) 213 // 214 // Below is a table describing the configuration for chunks for various 215 // heapAddrBits supported by the runtime. 216 // 217 // heapAddrBits | L1 Bits | L2 Bits | L2 Entry Size 218 // ------------------------------------------------ 219 // 32 | 0 | 10 | 128 KiB 220 // 33 (iOS) | 0 | 11 | 256 KiB 221 // 48 | 13 | 13 | 1 MiB 222 // 223 // There's no reason to use the L1 part of chunks on 32-bit, the 224 // address space is small so the L2 is small. For platforms with a 225 // 48-bit address space, we pick the L1 such that the L2 is 1 MiB 226 // in size, which is a good balance between low granularity without 227 // making the impact on BSS too high (note the L1 is stored directly 228 // in pageAlloc). 229 // 230 // To iterate over the bitmap, use inUse to determine which ranges 231 // are currently available. Otherwise one might iterate over unused 232 // ranges. 233 // 234 // Protected by mheapLock. 235 // 236 // TODO(mknyszek): Consider changing the definition of the bitmap 237 // such that 1 means free and 0 means in-use so that summaries and 238 // the bitmaps align better on zero-values. 239 chunks [1 << pallocChunksL1Bits]*[1 << pallocChunksL2Bits]pallocData 240 241 // The address to start an allocation search with. It must never 242 // point to any memory that is not contained in inUse, i.e. 243 // inUse.contains(searchAddr.addr()) must always be true. The one 244 // exception to this rule is that it may take on the value of 245 // maxOffAddr to indicate that the heap is exhausted. 246 // 247 // We guarantee that all valid heap addresses below this value 248 // are allocated and not worth searching. 249 searchAddr offAddr 250 251 // start and end represent the chunk indices 252 // which pageAlloc knows about. It assumes 253 // chunks in the range [start, end) are 254 // currently ready to use. 255 start, end chunkIdx 256 257 // inUse is a slice of ranges of address space which are 258 // known by the page allocator to be currently in-use (passed 259 // to grow). 260 // 261 // This field is currently unused on 32-bit architectures but 262 // is harmless to track. We care much more about having a 263 // contiguous heap in these cases and take additional measures 264 // to ensure that, so in nearly all cases this should have just 265 // 1 element. 266 // 267 // All access is protected by the mheapLock. 268 inUse addrRanges 269 270 _ uint32 // Align scav so it's easier to reason about alignment within scav. 271 272 // scav stores the scavenger state. 273 scav struct { 274 // index is an efficient index of chunks that have pages available to 275 // scavenge. 276 index scavengeIndex 277 278 // released is the amount of memory released this generation. 279 // 280 // Updated atomically. 281 released uintptr 282 283 _ uint32 // Align assistTime for atomics on 32-bit platforms. 284 285 // scavengeAssistTime is the time spent scavenging in the last GC cycle. 286 // 287 // This is reset once a GC cycle ends. 288 assistTime atomic.Int64 289 } 290 291 // mheap_.lock. This level of indirection makes it possible 292 // to test pageAlloc indepedently of the runtime allocator. 293 mheapLock *mutex 294 295 // sysStat is the runtime memstat to update when new system 296 // memory is committed by the pageAlloc for allocation metadata. 297 sysStat *sysMemStat 298 299 // summaryMappedReady is the number of bytes mapped in the Ready state 300 // in the summary structure. Used only for testing currently. 301 // 302 // Protected by mheapLock. 303 summaryMappedReady uintptr 304 305 // Whether or not this struct is being used in tests. 306 test bool 307 } 308 309 func (p *pageAlloc) init(mheapLock *mutex, sysStat *sysMemStat) { 310 if levelLogPages[0] > logMaxPackedValue { 311 // We can't represent 1<<levelLogPages[0] pages, the maximum number 312 // of pages we need to represent at the root level, in a summary, which 313 // is a big problem. Throw. 314 print("runtime: root level max pages = ", 1<<levelLogPages[0], "\n") 315 print("runtime: summary max pages = ", maxPackedValue, "\n") 316 throw("root level max pages doesn't fit in summary") 317 } 318 p.sysStat = sysStat 319 320 // Initialize p.inUse. 321 p.inUse.init(sysStat) 322 323 // System-dependent initialization. 324 p.sysInit() 325 326 // Start with the searchAddr in a state indicating there's no free memory. 327 p.searchAddr = maxSearchAddr() 328 329 // Set the mheapLock. 330 p.mheapLock = mheapLock 331 } 332 333 // tryChunkOf returns the bitmap data for the given chunk. 334 // 335 // Returns nil if the chunk data has not been mapped. 336 func (p *pageAlloc) tryChunkOf(ci chunkIdx) *pallocData { 337 l2 := p.chunks[ci.l1()] 338 if l2 == nil { 339 return nil 340 } 341 return &l2[ci.l2()] 342 } 343 344 // chunkOf returns the chunk at the given chunk index. 345 // 346 // The chunk index must be valid or this method may throw. 347 func (p *pageAlloc) chunkOf(ci chunkIdx) *pallocData { 348 return &p.chunks[ci.l1()][ci.l2()] 349 } 350 351 // grow sets up the metadata for the address range [base, base+size). 352 // It may allocate metadata, in which case *p.sysStat will be updated. 353 // 354 // p.mheapLock must be held. 355 func (p *pageAlloc) grow(base, size uintptr) { 356 assertLockHeld(p.mheapLock) 357 358 // Round up to chunks, since we can't deal with increments smaller 359 // than chunks. Also, sysGrow expects aligned values. 360 limit := alignUp(base+size, pallocChunkBytes) 361 base = alignDown(base, pallocChunkBytes) 362 363 // Grow the summary levels in a system-dependent manner. 364 // We just update a bunch of additional metadata here. 365 p.sysGrow(base, limit) 366 367 // Update p.start and p.end. 368 // If no growth happened yet, start == 0. This is generally 369 // safe since the zero page is unmapped. 370 firstGrowth := p.start == 0 371 start, end := chunkIndex(base), chunkIndex(limit) 372 if firstGrowth || start < p.start { 373 p.start = start 374 } 375 if end > p.end { 376 p.end = end 377 } 378 // Note that [base, limit) will never overlap with any existing 379 // range inUse because grow only ever adds never-used memory 380 // regions to the page allocator. 381 p.inUse.add(makeAddrRange(base, limit)) 382 383 // A grow operation is a lot like a free operation, so if our 384 // chunk ends up below p.searchAddr, update p.searchAddr to the 385 // new address, just like in free. 386 if b := (offAddr{base}); b.lessThan(p.searchAddr) { 387 p.searchAddr = b 388 } 389 390 // Add entries into chunks, which is sparse, if needed. Then, 391 // initialize the bitmap. 392 // 393 // Newly-grown memory is always considered scavenged. 394 // Set all the bits in the scavenged bitmaps high. 395 for c := chunkIndex(base); c < chunkIndex(limit); c++ { 396 if p.chunks[c.l1()] == nil { 397 // Create the necessary l2 entry. 398 // 399 // Store it atomically to avoid races with readers which 400 // don't acquire the heap lock. 401 r := sysAlloc(unsafe.Sizeof(*p.chunks[0]), p.sysStat) 402 if r == nil { 403 throw("pageAlloc: out of memory") 404 } 405 atomic.StorepNoWB(unsafe.Pointer(&p.chunks[c.l1()]), r) 406 } 407 p.chunkOf(c).scavenged.setRange(0, pallocChunkPages) 408 } 409 410 // Update summaries accordingly. The grow acts like a free, so 411 // we need to ensure this newly-free memory is visible in the 412 // summaries. 413 p.update(base, size/pageSize, true, false) 414 } 415 416 // update updates heap metadata. It must be called each time the bitmap 417 // is updated. 418 // 419 // If contig is true, update does some optimizations assuming that there was 420 // a contiguous allocation or free between addr and addr+npages. alloc indicates 421 // whether the operation performed was an allocation or a free. 422 // 423 // p.mheapLock must be held. 424 func (p *pageAlloc) update(base, npages uintptr, contig, alloc bool) { 425 assertLockHeld(p.mheapLock) 426 427 // base, limit, start, and end are inclusive. 428 limit := base + npages*pageSize - 1 429 sc, ec := chunkIndex(base), chunkIndex(limit) 430 431 // Handle updating the lowest level first. 432 if sc == ec { 433 // Fast path: the allocation doesn't span more than one chunk, 434 // so update this one and if the summary didn't change, return. 435 x := p.summary[len(p.summary)-1][sc] 436 y := p.chunkOf(sc).summarize() 437 if x == y { 438 return 439 } 440 p.summary[len(p.summary)-1][sc] = y 441 } else if contig { 442 // Slow contiguous path: the allocation spans more than one chunk 443 // and at least one summary is guaranteed to change. 444 summary := p.summary[len(p.summary)-1] 445 446 // Update the summary for chunk sc. 447 summary[sc] = p.chunkOf(sc).summarize() 448 449 // Update the summaries for chunks in between, which are 450 // either totally allocated or freed. 451 whole := p.summary[len(p.summary)-1][sc+1 : ec] 452 if alloc { 453 // Should optimize into a memclr. 454 for i := range whole { 455 whole[i] = 0 456 } 457 } else { 458 for i := range whole { 459 whole[i] = freeChunkSum 460 } 461 } 462 463 // Update the summary for chunk ec. 464 summary[ec] = p.chunkOf(ec).summarize() 465 } else { 466 // Slow general path: the allocation spans more than one chunk 467 // and at least one summary is guaranteed to change. 468 // 469 // We can't assume a contiguous allocation happened, so walk over 470 // every chunk in the range and manually recompute the summary. 471 summary := p.summary[len(p.summary)-1] 472 for c := sc; c <= ec; c++ { 473 summary[c] = p.chunkOf(c).summarize() 474 } 475 } 476 477 // Walk up the radix tree and update the summaries appropriately. 478 changed := true 479 for l := len(p.summary) - 2; l >= 0 && changed; l-- { 480 // Update summaries at level l from summaries at level l+1. 481 changed = false 482 483 // "Constants" for the previous level which we 484 // need to compute the summary from that level. 485 logEntriesPerBlock := levelBits[l+1] 486 logMaxPages := levelLogPages[l+1] 487 488 // lo and hi describe all the parts of the level we need to look at. 489 lo, hi := addrsToSummaryRange(l, base, limit+1) 490 491 // Iterate over each block, updating the corresponding summary in the less-granular level. 492 for i := lo; i < hi; i++ { 493 children := p.summary[l+1][i<<logEntriesPerBlock : (i+1)<<logEntriesPerBlock] 494 sum := mergeSummaries(children, logMaxPages) 495 old := p.summary[l][i] 496 if old != sum { 497 changed = true 498 p.summary[l][i] = sum 499 } 500 } 501 } 502 } 503 504 // allocRange marks the range of memory [base, base+npages*pageSize) as 505 // allocated. It also updates the summaries to reflect the newly-updated 506 // bitmap. 507 // 508 // Returns the amount of scavenged memory in bytes present in the 509 // allocated range. 510 // 511 // p.mheapLock must be held. 512 func (p *pageAlloc) allocRange(base, npages uintptr) uintptr { 513 assertLockHeld(p.mheapLock) 514 515 limit := base + npages*pageSize - 1 516 sc, ec := chunkIndex(base), chunkIndex(limit) 517 si, ei := chunkPageIndex(base), chunkPageIndex(limit) 518 519 scav := uint(0) 520 if sc == ec { 521 // The range doesn't cross any chunk boundaries. 522 chunk := p.chunkOf(sc) 523 scav += chunk.scavenged.popcntRange(si, ei+1-si) 524 chunk.allocRange(si, ei+1-si) 525 } else { 526 // The range crosses at least one chunk boundary. 527 chunk := p.chunkOf(sc) 528 scav += chunk.scavenged.popcntRange(si, pallocChunkPages-si) 529 chunk.allocRange(si, pallocChunkPages-si) 530 for c := sc + 1; c < ec; c++ { 531 chunk := p.chunkOf(c) 532 scav += chunk.scavenged.popcntRange(0, pallocChunkPages) 533 chunk.allocAll() 534 } 535 chunk = p.chunkOf(ec) 536 scav += chunk.scavenged.popcntRange(0, ei+1) 537 chunk.allocRange(0, ei+1) 538 } 539 p.update(base, npages, true, true) 540 return uintptr(scav) * pageSize 541 } 542 543 // findMappedAddr returns the smallest mapped offAddr that is 544 // >= addr. That is, if addr refers to mapped memory, then it is 545 // returned. If addr is higher than any mapped region, then 546 // it returns maxOffAddr. 547 // 548 // p.mheapLock must be held. 549 func (p *pageAlloc) findMappedAddr(addr offAddr) offAddr { 550 assertLockHeld(p.mheapLock) 551 552 // If we're not in a test, validate first by checking mheap_.arenas. 553 // This is a fast path which is only safe to use outside of testing. 554 ai := arenaIndex(addr.addr()) 555 if p.test || mheap_.arenas[ai.l1()] == nil || mheap_.arenas[ai.l1()][ai.l2()] == nil { 556 vAddr, ok := p.inUse.findAddrGreaterEqual(addr.addr()) 557 if ok { 558 return offAddr{vAddr} 559 } else { 560 // The candidate search address is greater than any 561 // known address, which means we definitely have no 562 // free memory left. 563 return maxOffAddr 564 } 565 } 566 return addr 567 } 568 569 // find searches for the first (address-ordered) contiguous free region of 570 // npages in size and returns a base address for that region. 571 // 572 // It uses p.searchAddr to prune its search and assumes that no palloc chunks 573 // below chunkIndex(p.searchAddr) contain any free memory at all. 574 // 575 // find also computes and returns a candidate p.searchAddr, which may or 576 // may not prune more of the address space than p.searchAddr already does. 577 // This candidate is always a valid p.searchAddr. 578 // 579 // find represents the slow path and the full radix tree search. 580 // 581 // Returns a base address of 0 on failure, in which case the candidate 582 // searchAddr returned is invalid and must be ignored. 583 // 584 // p.mheapLock must be held. 585 func (p *pageAlloc) find(npages uintptr) (uintptr, offAddr) { 586 assertLockHeld(p.mheapLock) 587 588 // Search algorithm. 589 // 590 // This algorithm walks each level l of the radix tree from the root level 591 // to the leaf level. It iterates over at most 1 << levelBits[l] of entries 592 // in a given level in the radix tree, and uses the summary information to 593 // find either: 594 // 1) That a given subtree contains a large enough contiguous region, at 595 // which point it continues iterating on the next level, or 596 // 2) That there are enough contiguous boundary-crossing bits to satisfy 597 // the allocation, at which point it knows exactly where to start 598 // allocating from. 599 // 600 // i tracks the index into the current level l's structure for the 601 // contiguous 1 << levelBits[l] entries we're actually interested in. 602 // 603 // NOTE: Technically this search could allocate a region which crosses 604 // the arenaBaseOffset boundary, which when arenaBaseOffset != 0, is 605 // a discontinuity. However, the only way this could happen is if the 606 // page at the zero address is mapped, and this is impossible on 607 // every system we support where arenaBaseOffset != 0. So, the 608 // discontinuity is already encoded in the fact that the OS will never 609 // map the zero page for us, and this function doesn't try to handle 610 // this case in any way. 611 612 // i is the beginning of the block of entries we're searching at the 613 // current level. 614 i := 0 615 616 // firstFree is the region of address space that we are certain to 617 // find the first free page in the heap. base and bound are the inclusive 618 // bounds of this window, and both are addresses in the linearized, contiguous 619 // view of the address space (with arenaBaseOffset pre-added). At each level, 620 // this window is narrowed as we find the memory region containing the 621 // first free page of memory. To begin with, the range reflects the 622 // full process address space. 623 // 624 // firstFree is updated by calling foundFree each time free space in the 625 // heap is discovered. 626 // 627 // At the end of the search, base.addr() is the best new 628 // searchAddr we could deduce in this search. 629 firstFree := struct { 630 base, bound offAddr 631 }{ 632 base: minOffAddr, 633 bound: maxOffAddr, 634 } 635 // foundFree takes the given address range [addr, addr+size) and 636 // updates firstFree if it is a narrower range. The input range must 637 // either be fully contained within firstFree or not overlap with it 638 // at all. 639 // 640 // This way, we'll record the first summary we find with any free 641 // pages on the root level and narrow that down if we descend into 642 // that summary. But as soon as we need to iterate beyond that summary 643 // in a level to find a large enough range, we'll stop narrowing. 644 foundFree := func(addr offAddr, size uintptr) { 645 if firstFree.base.lessEqual(addr) && addr.add(size-1).lessEqual(firstFree.bound) { 646 // This range fits within the current firstFree window, so narrow 647 // down the firstFree window to the base and bound of this range. 648 firstFree.base = addr 649 firstFree.bound = addr.add(size - 1) 650 } else if !(addr.add(size-1).lessThan(firstFree.base) || firstFree.bound.lessThan(addr)) { 651 // This range only partially overlaps with the firstFree range, 652 // so throw. 653 print("runtime: addr = ", hex(addr.addr()), ", size = ", size, "\n") 654 print("runtime: base = ", hex(firstFree.base.addr()), ", bound = ", hex(firstFree.bound.addr()), "\n") 655 throw("range partially overlaps") 656 } 657 } 658 659 // lastSum is the summary which we saw on the previous level that made us 660 // move on to the next level. Used to print additional information in the 661 // case of a catastrophic failure. 662 // lastSumIdx is that summary's index in the previous level. 663 lastSum := packPallocSum(0, 0, 0) 664 lastSumIdx := -1 665 666 nextLevel: 667 for l := 0; l < len(p.summary); l++ { 668 // For the root level, entriesPerBlock is the whole level. 669 entriesPerBlock := 1 << levelBits[l] 670 logMaxPages := levelLogPages[l] 671 672 // We've moved into a new level, so let's update i to our new 673 // starting index. This is a no-op for level 0. 674 i <<= levelBits[l] 675 676 // Slice out the block of entries we care about. 677 entries := p.summary[l][i : i+entriesPerBlock] 678 679 // Determine j0, the first index we should start iterating from. 680 // The searchAddr may help us eliminate iterations if we followed the 681 // searchAddr on the previous level or we're on the root leve, in which 682 // case the searchAddr should be the same as i after levelShift. 683 j0 := 0 684 if searchIdx := offAddrToLevelIndex(l, p.searchAddr); searchIdx&^(entriesPerBlock-1) == i { 685 j0 = searchIdx & (entriesPerBlock - 1) 686 } 687 688 // Run over the level entries looking for 689 // a contiguous run of at least npages either 690 // within an entry or across entries. 691 // 692 // base contains the page index (relative to 693 // the first entry's first page) of the currently 694 // considered run of consecutive pages. 695 // 696 // size contains the size of the currently considered 697 // run of consecutive pages. 698 var base, size uint 699 for j := j0; j < len(entries); j++ { 700 sum := entries[j] 701 if sum == 0 { 702 // A full entry means we broke any streak and 703 // that we should skip it altogether. 704 size = 0 705 continue 706 } 707 708 // We've encountered a non-zero summary which means 709 // free memory, so update firstFree. 710 foundFree(levelIndexToOffAddr(l, i+j), (uintptr(1)<<logMaxPages)*pageSize) 711 712 s := sum.start() 713 if size+s >= uint(npages) { 714 // If size == 0 we don't have a run yet, 715 // which means base isn't valid. So, set 716 // base to the first page in this block. 717 if size == 0 { 718 base = uint(j) << logMaxPages 719 } 720 // We hit npages; we're done! 721 size += s 722 break 723 } 724 if sum.max() >= uint(npages) { 725 // The entry itself contains npages contiguous 726 // free pages, so continue on the next level 727 // to find that run. 728 i += j 729 lastSumIdx = i 730 lastSum = sum 731 continue nextLevel 732 } 733 if size == 0 || s < 1<<logMaxPages { 734 // We either don't have a current run started, or this entry 735 // isn't totally free (meaning we can't continue the current 736 // one), so try to begin a new run by setting size and base 737 // based on sum.end. 738 size = sum.end() 739 base = uint(j+1)<<logMaxPages - size 740 continue 741 } 742 // The entry is completely free, so continue the run. 743 size += 1 << logMaxPages 744 } 745 if size >= uint(npages) { 746 // We found a sufficiently large run of free pages straddling 747 // some boundary, so compute the address and return it. 748 addr := levelIndexToOffAddr(l, i).add(uintptr(base) * pageSize).addr() 749 return addr, p.findMappedAddr(firstFree.base) 750 } 751 if l == 0 { 752 // We're at level zero, so that means we've exhausted our search. 753 return 0, maxSearchAddr() 754 } 755 756 // We're not at level zero, and we exhausted the level we were looking in. 757 // This means that either our calculations were wrong or the level above 758 // lied to us. In either case, dump some useful state and throw. 759 print("runtime: summary[", l-1, "][", lastSumIdx, "] = ", lastSum.start(), ", ", lastSum.max(), ", ", lastSum.end(), "\n") 760 print("runtime: level = ", l, ", npages = ", npages, ", j0 = ", j0, "\n") 761 print("runtime: p.searchAddr = ", hex(p.searchAddr.addr()), ", i = ", i, "\n") 762 print("runtime: levelShift[level] = ", levelShift[l], ", levelBits[level] = ", levelBits[l], "\n") 763 for j := 0; j < len(entries); j++ { 764 sum := entries[j] 765 print("runtime: summary[", l, "][", i+j, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") 766 } 767 throw("bad summary data") 768 } 769 770 // Since we've gotten to this point, that means we haven't found a 771 // sufficiently-sized free region straddling some boundary (chunk or larger). 772 // This means the last summary we inspected must have had a large enough "max" 773 // value, so look inside the chunk to find a suitable run. 774 // 775 // After iterating over all levels, i must contain a chunk index which 776 // is what the final level represents. 777 ci := chunkIdx(i) 778 j, searchIdx := p.chunkOf(ci).find(npages, 0) 779 if j == ^uint(0) { 780 // We couldn't find any space in this chunk despite the summaries telling 781 // us it should be there. There's likely a bug, so dump some state and throw. 782 sum := p.summary[len(p.summary)-1][i] 783 print("runtime: summary[", len(p.summary)-1, "][", i, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") 784 print("runtime: npages = ", npages, "\n") 785 throw("bad summary data") 786 } 787 788 // Compute the address at which the free space starts. 789 addr := chunkBase(ci) + uintptr(j)*pageSize 790 791 // Since we actually searched the chunk, we may have 792 // found an even narrower free window. 793 searchAddr := chunkBase(ci) + uintptr(searchIdx)*pageSize 794 foundFree(offAddr{searchAddr}, chunkBase(ci+1)-searchAddr) 795 return addr, p.findMappedAddr(firstFree.base) 796 } 797 798 // alloc allocates npages worth of memory from the page heap, returning the base 799 // address for the allocation and the amount of scavenged memory in bytes 800 // contained in the region [base address, base address + npages*pageSize). 801 // 802 // Returns a 0 base address on failure, in which case other returned values 803 // should be ignored. 804 // 805 // p.mheapLock must be held. 806 // 807 // Must run on the system stack because p.mheapLock must be held. 808 // 809 //go:systemstack 810 func (p *pageAlloc) alloc(npages uintptr) (addr uintptr, scav uintptr) { 811 assertLockHeld(p.mheapLock) 812 813 // If the searchAddr refers to a region which has a higher address than 814 // any known chunk, then we know we're out of memory. 815 if chunkIndex(p.searchAddr.addr()) >= p.end { 816 return 0, 0 817 } 818 819 // If npages has a chance of fitting in the chunk where the searchAddr is, 820 // search it directly. 821 searchAddr := minOffAddr 822 if pallocChunkPages-chunkPageIndex(p.searchAddr.addr()) >= uint(npages) { 823 // npages is guaranteed to be no greater than pallocChunkPages here. 824 i := chunkIndex(p.searchAddr.addr()) 825 if max := p.summary[len(p.summary)-1][i].max(); max >= uint(npages) { 826 j, searchIdx := p.chunkOf(i).find(npages, chunkPageIndex(p.searchAddr.addr())) 827 if j == ^uint(0) { 828 print("runtime: max = ", max, ", npages = ", npages, "\n") 829 print("runtime: searchIdx = ", chunkPageIndex(p.searchAddr.addr()), ", p.searchAddr = ", hex(p.searchAddr.addr()), "\n") 830 throw("bad summary data") 831 } 832 addr = chunkBase(i) + uintptr(j)*pageSize 833 searchAddr = offAddr{chunkBase(i) + uintptr(searchIdx)*pageSize} 834 goto Found 835 } 836 } 837 // We failed to use a searchAddr for one reason or another, so try 838 // the slow path. 839 addr, searchAddr = p.find(npages) 840 if addr == 0 { 841 if npages == 1 { 842 // We failed to find a single free page, the smallest unit 843 // of allocation. This means we know the heap is completely 844 // exhausted. Otherwise, the heap still might have free 845 // space in it, just not enough contiguous space to 846 // accommodate npages. 847 p.searchAddr = maxSearchAddr() 848 } 849 return 0, 0 850 } 851 Found: 852 // Go ahead and actually mark the bits now that we have an address. 853 scav = p.allocRange(addr, npages) 854 855 // If we found a higher searchAddr, we know that all the 856 // heap memory before that searchAddr in an offset address space is 857 // allocated, so bump p.searchAddr up to the new one. 858 if p.searchAddr.lessThan(searchAddr) { 859 p.searchAddr = searchAddr 860 } 861 return addr, scav 862 } 863 864 // free returns npages worth of memory starting at base back to the page heap. 865 // 866 // p.mheapLock must be held. 867 // 868 // Must run on the system stack because p.mheapLock must be held. 869 // 870 //go:systemstack 871 func (p *pageAlloc) free(base, npages uintptr, scavenged bool) { 872 assertLockHeld(p.mheapLock) 873 874 // If we're freeing pages below the p.searchAddr, update searchAddr. 875 if b := (offAddr{base}); b.lessThan(p.searchAddr) { 876 p.searchAddr = b 877 } 878 limit := base + npages*pageSize - 1 879 if !scavenged { 880 p.scav.index.mark(base, limit+1) 881 } 882 if npages == 1 { 883 // Fast path: we're clearing a single bit, and we know exactly 884 // where it is, so mark it directly. 885 i := chunkIndex(base) 886 p.chunkOf(i).free1(chunkPageIndex(base)) 887 } else { 888 // Slow path: we're clearing more bits so we may need to iterate. 889 sc, ec := chunkIndex(base), chunkIndex(limit) 890 si, ei := chunkPageIndex(base), chunkPageIndex(limit) 891 892 if sc == ec { 893 // The range doesn't cross any chunk boundaries. 894 p.chunkOf(sc).free(si, ei+1-si) 895 } else { 896 // The range crosses at least one chunk boundary. 897 p.chunkOf(sc).free(si, pallocChunkPages-si) 898 for c := sc + 1; c < ec; c++ { 899 p.chunkOf(c).freeAll() 900 } 901 p.chunkOf(ec).free(0, ei+1) 902 } 903 } 904 p.update(base, npages, true, false) 905 } 906 907 const ( 908 pallocSumBytes = unsafe.Sizeof(pallocSum(0)) 909 910 // maxPackedValue is the maximum value that any of the three fields in 911 // the pallocSum may take on. 912 maxPackedValue = 1 << logMaxPackedValue 913 logMaxPackedValue = logPallocChunkPages + (summaryLevels-1)*summaryLevelBits 914 915 freeChunkSum = pallocSum(uint64(pallocChunkPages) | 916 uint64(pallocChunkPages<<logMaxPackedValue) | 917 uint64(pallocChunkPages<<(2*logMaxPackedValue))) 918 ) 919 920 // pallocSum is a packed summary type which packs three numbers: start, max, 921 // and end into a single 8-byte value. Each of these values are a summary of 922 // a bitmap and are thus counts, each of which may have a maximum value of 923 // 2^21 - 1, or all three may be equal to 2^21. The latter case is represented 924 // by just setting the 64th bit. 925 type pallocSum uint64 926 927 // packPallocSum takes a start, max, and end value and produces a pallocSum. 928 func packPallocSum(start, max, end uint) pallocSum { 929 if max == maxPackedValue { 930 return pallocSum(uint64(1 << 63)) 931 } 932 return pallocSum((uint64(start) & (maxPackedValue - 1)) | 933 ((uint64(max) & (maxPackedValue - 1)) << logMaxPackedValue) | 934 ((uint64(end) & (maxPackedValue - 1)) << (2 * logMaxPackedValue))) 935 } 936 937 // start extracts the start value from a packed sum. 938 func (p pallocSum) start() uint { 939 if uint64(p)&uint64(1<<63) != 0 { 940 return maxPackedValue 941 } 942 return uint(uint64(p) & (maxPackedValue - 1)) 943 } 944 945 // max extracts the max value from a packed sum. 946 func (p pallocSum) max() uint { 947 if uint64(p)&uint64(1<<63) != 0 { 948 return maxPackedValue 949 } 950 return uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)) 951 } 952 953 // end extracts the end value from a packed sum. 954 func (p pallocSum) end() uint { 955 if uint64(p)&uint64(1<<63) != 0 { 956 return maxPackedValue 957 } 958 return uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) 959 } 960 961 // unpack unpacks all three values from the summary. 962 func (p pallocSum) unpack() (uint, uint, uint) { 963 if uint64(p)&uint64(1<<63) != 0 { 964 return maxPackedValue, maxPackedValue, maxPackedValue 965 } 966 return uint(uint64(p) & (maxPackedValue - 1)), 967 uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)), 968 uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) 969 } 970 971 // mergeSummaries merges consecutive summaries which may each represent at 972 // most 1 << logMaxPagesPerSum pages each together into one. 973 func mergeSummaries(sums []pallocSum, logMaxPagesPerSum uint) pallocSum { 974 // Merge the summaries in sums into one. 975 // 976 // We do this by keeping a running summary representing the merged 977 // summaries of sums[:i] in start, max, and end. 978 start, max, end := sums[0].unpack() 979 for i := 1; i < len(sums); i++ { 980 // Merge in sums[i]. 981 si, mi, ei := sums[i].unpack() 982 983 // Merge in sums[i].start only if the running summary is 984 // completely free, otherwise this summary's start 985 // plays no role in the combined sum. 986 if start == uint(i)<<logMaxPagesPerSum { 987 start += si 988 } 989 990 // Recompute the max value of the running sum by looking 991 // across the boundary between the running sum and sums[i] 992 // and at the max sums[i], taking the greatest of those two 993 // and the max of the running sum. 994 if end+si > max { 995 max = end + si 996 } 997 if mi > max { 998 max = mi 999 } 1000 1001 // Merge in end by checking if this new summary is totally 1002 // free. If it is, then we want to extend the running sum's 1003 // end by the new summary. If not, then we have some alloc'd 1004 // pages in there and we just want to take the end value in 1005 // sums[i]. 1006 if ei == 1<<logMaxPagesPerSum { 1007 end += 1 << logMaxPagesPerSum 1008 } else { 1009 end = ei 1010 } 1011 } 1012 return packPallocSum(start, max, end) 1013 } 1014