github.com/mtsmfm/go/src@v0.0.0-20221020090648-44bdcb9f8fde/runtime/mpagealloc.go (about) 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 "unsafe" 52 ) 53 54 const ( 55 // The size of a bitmap chunk, i.e. the amount of bits (that is, pages) to consider 56 // in the bitmap at once. 57 pallocChunkPages = 1 << logPallocChunkPages 58 pallocChunkBytes = pallocChunkPages * pageSize 59 logPallocChunkPages = 9 60 logPallocChunkBytes = logPallocChunkPages + pageShift 61 62 // The number of radix bits for each level. 63 // 64 // The value of 3 is chosen such that the block of summaries we need to scan at 65 // each level fits in 64 bytes (2^3 summaries * 8 bytes per summary), which is 66 // close to the L1 cache line width on many systems. Also, a value of 3 fits 4 tree 67 // levels perfectly into the 21-bit pallocBits summary field at the root level. 68 // 69 // The following equation explains how each of the constants relate: 70 // summaryL0Bits + (summaryLevels-1)*summaryLevelBits + logPallocChunkBytes = heapAddrBits 71 // 72 // summaryLevels is an architecture-dependent value defined in mpagealloc_*.go. 73 summaryLevelBits = 3 74 summaryL0Bits = heapAddrBits - logPallocChunkBytes - (summaryLevels-1)*summaryLevelBits 75 76 // pallocChunksL2Bits is the number of bits of the chunk index number 77 // covered by the second level of the chunks map. 78 // 79 // See (*pageAlloc).chunks for more details. Update the documentation 80 // there should this change. 81 pallocChunksL2Bits = heapAddrBits - logPallocChunkBytes - pallocChunksL1Bits 82 pallocChunksL1Shift = pallocChunksL2Bits 83 ) 84 85 // maxSearchAddr returns the maximum searchAddr value, which indicates 86 // that the heap has no free space. 87 // 88 // This function exists just to make it clear that this is the maximum address 89 // for the page allocator's search space. See maxOffAddr for details. 90 // 91 // It's a function (rather than a variable) because it needs to be 92 // usable before package runtime's dynamic initialization is complete. 93 // See #51913 for details. 94 func maxSearchAddr() offAddr { return maxOffAddr } 95 96 // Global chunk index. 97 // 98 // Represents an index into the leaf level of the radix tree. 99 // Similar to arenaIndex, except instead of arenas, it divides the address 100 // space into chunks. 101 type chunkIdx uint 102 103 // chunkIndex returns the global index of the palloc chunk containing the 104 // pointer p. 105 func chunkIndex(p uintptr) chunkIdx { 106 return chunkIdx((p - arenaBaseOffset) / pallocChunkBytes) 107 } 108 109 // chunkIndex returns the base address of the palloc chunk at index ci. 110 func chunkBase(ci chunkIdx) uintptr { 111 return uintptr(ci)*pallocChunkBytes + arenaBaseOffset 112 } 113 114 // chunkPageIndex computes the index of the page that contains p, 115 // relative to the chunk which contains p. 116 func chunkPageIndex(p uintptr) uint { 117 return uint(p % pallocChunkBytes / pageSize) 118 } 119 120 // l1 returns the index into the first level of (*pageAlloc).chunks. 121 func (i chunkIdx) l1() uint { 122 if pallocChunksL1Bits == 0 { 123 // Let the compiler optimize this away if there's no 124 // L1 map. 125 return 0 126 } else { 127 return uint(i) >> pallocChunksL1Shift 128 } 129 } 130 131 // l2 returns the index into the second level of (*pageAlloc).chunks. 132 func (i chunkIdx) l2() uint { 133 if pallocChunksL1Bits == 0 { 134 return uint(i) 135 } else { 136 return uint(i) & (1<<pallocChunksL2Bits - 1) 137 } 138 } 139 140 // offAddrToLevelIndex converts an address in the offset address space 141 // to the index into summary[level] containing addr. 142 func offAddrToLevelIndex(level int, addr offAddr) int { 143 return int((addr.a - arenaBaseOffset) >> levelShift[level]) 144 } 145 146 // levelIndexToOffAddr converts an index into summary[level] into 147 // the corresponding address in the offset address space. 148 func levelIndexToOffAddr(level, idx int) offAddr { 149 return offAddr{(uintptr(idx) << levelShift[level]) + arenaBaseOffset} 150 } 151 152 // addrsToSummaryRange converts base and limit pointers into a range 153 // of entries for the given summary level. 154 // 155 // The returned range is inclusive on the lower bound and exclusive on 156 // the upper bound. 157 func addrsToSummaryRange(level int, base, limit uintptr) (lo int, hi int) { 158 // This is slightly more nuanced than just a shift for the exclusive 159 // upper-bound. Note that the exclusive upper bound may be within a 160 // summary at this level, meaning if we just do the obvious computation 161 // hi will end up being an inclusive upper bound. Unfortunately, just 162 // adding 1 to that is too broad since we might be on the very edge 163 // of a summary's max page count boundary for this level 164 // (1 << levelLogPages[level]). So, make limit an inclusive upper bound 165 // then shift, then add 1, so we get an exclusive upper bound at the end. 166 lo = int((base - arenaBaseOffset) >> levelShift[level]) 167 hi = int(((limit-1)-arenaBaseOffset)>>levelShift[level]) + 1 168 return 169 } 170 171 // blockAlignSummaryRange aligns indices into the given level to that 172 // level's block width (1 << levelBits[level]). It assumes lo is inclusive 173 // and hi is exclusive, and so aligns them down and up respectively. 174 func blockAlignSummaryRange(level int, lo, hi int) (int, int) { 175 e := uintptr(1) << levelBits[level] 176 return int(alignDown(uintptr(lo), e)), int(alignUp(uintptr(hi), e)) 177 } 178 179 type pageAlloc struct { 180 // Radix tree of summaries. 181 // 182 // Each slice's cap represents the whole memory reservation. 183 // Each slice's len reflects the allocator's maximum known 184 // mapped heap address for that level. 185 // 186 // The backing store of each summary level is reserved in init 187 // and may or may not be committed in grow (small address spaces 188 // may commit all the memory in init). 189 // 190 // The purpose of keeping len <= cap is to enforce bounds checks 191 // on the top end of the slice so that instead of an unknown 192 // runtime segmentation fault, we get a much friendlier out-of-bounds 193 // error. 194 // 195 // To iterate over a summary level, use inUse to determine which ranges 196 // are currently available. Otherwise one might try to access 197 // memory which is only Reserved which may result in a hard fault. 198 // 199 // We may still get segmentation faults < len since some of that 200 // memory may not be committed yet. 201 summary [summaryLevels][]pallocSum 202 203 // chunks is a slice of bitmap chunks. 204 // 205 // The total size of chunks is quite large on most 64-bit platforms 206 // (O(GiB) or more) if flattened, so rather than making one large mapping 207 // (which has problems on some platforms, even when PROT_NONE) we use a 208 // two-level sparse array approach similar to the arena index in mheap. 209 // 210 // To find the chunk containing a memory address `a`, do: 211 // chunkOf(chunkIndex(a)) 212 // 213 // Below is a table describing the configuration for chunks for various 214 // heapAddrBits supported by the runtime. 215 // 216 // heapAddrBits | L1 Bits | L2 Bits | L2 Entry Size 217 // ------------------------------------------------ 218 // 32 | 0 | 10 | 128 KiB 219 // 33 (iOS) | 0 | 11 | 256 KiB 220 // 48 | 13 | 13 | 1 MiB 221 // 222 // There's no reason to use the L1 part of chunks on 32-bit, the 223 // address space is small so the L2 is small. For platforms with a 224 // 48-bit address space, we pick the L1 such that the L2 is 1 MiB 225 // in size, which is a good balance between low granularity without 226 // making the impact on BSS too high (note the L1 is stored directly 227 // in pageAlloc). 228 // 229 // To iterate over the bitmap, use inUse to determine which ranges 230 // are currently available. Otherwise one might iterate over unused 231 // ranges. 232 // 233 // Protected by mheapLock. 234 // 235 // TODO(mknyszek): Consider changing the definition of the bitmap 236 // such that 1 means free and 0 means in-use so that summaries and 237 // the bitmaps align better on zero-values. 238 chunks [1 << pallocChunksL1Bits]*[1 << pallocChunksL2Bits]pallocData 239 240 // The address to start an allocation search with. It must never 241 // point to any memory that is not contained in inUse, i.e. 242 // inUse.contains(searchAddr.addr()) must always be true. The one 243 // exception to this rule is that it may take on the value of 244 // maxOffAddr to indicate that the heap is exhausted. 245 // 246 // We guarantee that all valid heap addresses below this value 247 // are allocated and not worth searching. 248 searchAddr offAddr 249 250 // start and end represent the chunk indices 251 // which pageAlloc knows about. It assumes 252 // chunks in the range [start, end) are 253 // currently ready to use. 254 start, end chunkIdx 255 256 // inUse is a slice of ranges of address space which are 257 // known by the page allocator to be currently in-use (passed 258 // to grow). 259 // 260 // This field is currently unused on 32-bit architectures but 261 // is harmless to track. We care much more about having a 262 // contiguous heap in these cases and take additional measures 263 // to ensure that, so in nearly all cases this should have just 264 // 1 element. 265 // 266 // All access is protected by the mheapLock. 267 inUse addrRanges 268 269 // scav stores the scavenger state. 270 scav struct { 271 // index is an efficient index of chunks that have pages available to 272 // scavenge. 273 index scavengeIndex 274 275 // released is the amount of memory released this scavenge cycle. 276 // 277 // Updated atomically. 278 released uintptr 279 } 280 281 // mheap_.lock. This level of indirection makes it possible 282 // to test pageAlloc indepedently of the runtime allocator. 283 mheapLock *mutex 284 285 // sysStat is the runtime memstat to update when new system 286 // memory is committed by the pageAlloc for allocation metadata. 287 sysStat *sysMemStat 288 289 // summaryMappedReady is the number of bytes mapped in the Ready state 290 // in the summary structure. Used only for testing currently. 291 // 292 // Protected by mheapLock. 293 summaryMappedReady uintptr 294 295 // Whether or not this struct is being used in tests. 296 test bool 297 } 298 299 func (p *pageAlloc) init(mheapLock *mutex, sysStat *sysMemStat) { 300 if levelLogPages[0] > logMaxPackedValue { 301 // We can't represent 1<<levelLogPages[0] pages, the maximum number 302 // of pages we need to represent at the root level, in a summary, which 303 // is a big problem. Throw. 304 print("runtime: root level max pages = ", 1<<levelLogPages[0], "\n") 305 print("runtime: summary max pages = ", maxPackedValue, "\n") 306 throw("root level max pages doesn't fit in summary") 307 } 308 p.sysStat = sysStat 309 310 // Initialize p.inUse. 311 p.inUse.init(sysStat) 312 313 // System-dependent initialization. 314 p.sysInit() 315 316 // Start with the searchAddr in a state indicating there's no free memory. 317 p.searchAddr = maxSearchAddr() 318 319 // Set the mheapLock. 320 p.mheapLock = mheapLock 321 } 322 323 // tryChunkOf returns the bitmap data for the given chunk. 324 // 325 // Returns nil if the chunk data has not been mapped. 326 func (p *pageAlloc) tryChunkOf(ci chunkIdx) *pallocData { 327 l2 := p.chunks[ci.l1()] 328 if l2 == nil { 329 return nil 330 } 331 return &l2[ci.l2()] 332 } 333 334 // chunkOf returns the chunk at the given chunk index. 335 // 336 // The chunk index must be valid or this method may throw. 337 func (p *pageAlloc) chunkOf(ci chunkIdx) *pallocData { 338 return &p.chunks[ci.l1()][ci.l2()] 339 } 340 341 // grow sets up the metadata for the address range [base, base+size). 342 // It may allocate metadata, in which case *p.sysStat will be updated. 343 // 344 // p.mheapLock must be held. 345 func (p *pageAlloc) grow(base, size uintptr) { 346 assertLockHeld(p.mheapLock) 347 348 // Round up to chunks, since we can't deal with increments smaller 349 // than chunks. Also, sysGrow expects aligned values. 350 limit := alignUp(base+size, pallocChunkBytes) 351 base = alignDown(base, pallocChunkBytes) 352 353 // Grow the summary levels in a system-dependent manner. 354 // We just update a bunch of additional metadata here. 355 p.sysGrow(base, limit) 356 357 // Update p.start and p.end. 358 // If no growth happened yet, start == 0. This is generally 359 // safe since the zero page is unmapped. 360 firstGrowth := p.start == 0 361 start, end := chunkIndex(base), chunkIndex(limit) 362 if firstGrowth || start < p.start { 363 p.start = start 364 } 365 if end > p.end { 366 p.end = end 367 } 368 // Note that [base, limit) will never overlap with any existing 369 // range inUse because grow only ever adds never-used memory 370 // regions to the page allocator. 371 p.inUse.add(makeAddrRange(base, limit)) 372 373 // A grow operation is a lot like a free operation, so if our 374 // chunk ends up below p.searchAddr, update p.searchAddr to the 375 // new address, just like in free. 376 if b := (offAddr{base}); b.lessThan(p.searchAddr) { 377 p.searchAddr = b 378 } 379 380 // Add entries into chunks, which is sparse, if needed. Then, 381 // initialize the bitmap. 382 // 383 // Newly-grown memory is always considered scavenged. 384 // Set all the bits in the scavenged bitmaps high. 385 for c := chunkIndex(base); c < chunkIndex(limit); c++ { 386 if p.chunks[c.l1()] == nil { 387 // Create the necessary l2 entry. 388 r := sysAlloc(unsafe.Sizeof(*p.chunks[0]), p.sysStat) 389 if r == nil { 390 throw("pageAlloc: out of memory") 391 } 392 // Store the new chunk block but avoid a write barrier. 393 // grow is used in call chains that disallow write barriers. 394 *(*uintptr)(unsafe.Pointer(&p.chunks[c.l1()])) = uintptr(r) 395 } 396 p.chunkOf(c).scavenged.setRange(0, pallocChunkPages) 397 } 398 399 // Update summaries accordingly. The grow acts like a free, so 400 // we need to ensure this newly-free memory is visible in the 401 // summaries. 402 p.update(base, size/pageSize, true, false) 403 } 404 405 // update updates heap metadata. It must be called each time the bitmap 406 // is updated. 407 // 408 // If contig is true, update does some optimizations assuming that there was 409 // a contiguous allocation or free between addr and addr+npages. alloc indicates 410 // whether the operation performed was an allocation or a free. 411 // 412 // p.mheapLock must be held. 413 func (p *pageAlloc) update(base, npages uintptr, contig, alloc bool) { 414 assertLockHeld(p.mheapLock) 415 416 // base, limit, start, and end are inclusive. 417 limit := base + npages*pageSize - 1 418 sc, ec := chunkIndex(base), chunkIndex(limit) 419 420 // Handle updating the lowest level first. 421 if sc == ec { 422 // Fast path: the allocation doesn't span more than one chunk, 423 // so update this one and if the summary didn't change, return. 424 x := p.summary[len(p.summary)-1][sc] 425 y := p.chunkOf(sc).summarize() 426 if x == y { 427 return 428 } 429 p.summary[len(p.summary)-1][sc] = y 430 } else if contig { 431 // Slow contiguous path: the allocation spans more than one chunk 432 // and at least one summary is guaranteed to change. 433 summary := p.summary[len(p.summary)-1] 434 435 // Update the summary for chunk sc. 436 summary[sc] = p.chunkOf(sc).summarize() 437 438 // Update the summaries for chunks in between, which are 439 // either totally allocated or freed. 440 whole := p.summary[len(p.summary)-1][sc+1 : ec] 441 if alloc { 442 // Should optimize into a memclr. 443 for i := range whole { 444 whole[i] = 0 445 } 446 } else { 447 for i := range whole { 448 whole[i] = freeChunkSum 449 } 450 } 451 452 // Update the summary for chunk ec. 453 summary[ec] = p.chunkOf(ec).summarize() 454 } else { 455 // Slow general path: the allocation spans more than one chunk 456 // and at least one summary is guaranteed to change. 457 // 458 // We can't assume a contiguous allocation happened, so walk over 459 // every chunk in the range and manually recompute the summary. 460 summary := p.summary[len(p.summary)-1] 461 for c := sc; c <= ec; c++ { 462 summary[c] = p.chunkOf(c).summarize() 463 } 464 } 465 466 // Walk up the radix tree and update the summaries appropriately. 467 changed := true 468 for l := len(p.summary) - 2; l >= 0 && changed; l-- { 469 // Update summaries at level l from summaries at level l+1. 470 changed = false 471 472 // "Constants" for the previous level which we 473 // need to compute the summary from that level. 474 logEntriesPerBlock := levelBits[l+1] 475 logMaxPages := levelLogPages[l+1] 476 477 // lo and hi describe all the parts of the level we need to look at. 478 lo, hi := addrsToSummaryRange(l, base, limit+1) 479 480 // Iterate over each block, updating the corresponding summary in the less-granular level. 481 for i := lo; i < hi; i++ { 482 children := p.summary[l+1][i<<logEntriesPerBlock : (i+1)<<logEntriesPerBlock] 483 sum := mergeSummaries(children, logMaxPages) 484 old := p.summary[l][i] 485 if old != sum { 486 changed = true 487 p.summary[l][i] = sum 488 } 489 } 490 } 491 } 492 493 // allocRange marks the range of memory [base, base+npages*pageSize) as 494 // allocated. It also updates the summaries to reflect the newly-updated 495 // bitmap. 496 // 497 // Returns the amount of scavenged memory in bytes present in the 498 // allocated range. 499 // 500 // p.mheapLock must be held. 501 func (p *pageAlloc) allocRange(base, npages uintptr) uintptr { 502 assertLockHeld(p.mheapLock) 503 504 limit := base + npages*pageSize - 1 505 sc, ec := chunkIndex(base), chunkIndex(limit) 506 si, ei := chunkPageIndex(base), chunkPageIndex(limit) 507 508 scav := uint(0) 509 if sc == ec { 510 // The range doesn't cross any chunk boundaries. 511 chunk := p.chunkOf(sc) 512 scav += chunk.scavenged.popcntRange(si, ei+1-si) 513 chunk.allocRange(si, ei+1-si) 514 } else { 515 // The range crosses at least one chunk boundary. 516 chunk := p.chunkOf(sc) 517 scav += chunk.scavenged.popcntRange(si, pallocChunkPages-si) 518 chunk.allocRange(si, pallocChunkPages-si) 519 for c := sc + 1; c < ec; c++ { 520 chunk := p.chunkOf(c) 521 scav += chunk.scavenged.popcntRange(0, pallocChunkPages) 522 chunk.allocAll() 523 } 524 chunk = p.chunkOf(ec) 525 scav += chunk.scavenged.popcntRange(0, ei+1) 526 chunk.allocRange(0, ei+1) 527 } 528 p.update(base, npages, true, true) 529 return uintptr(scav) * pageSize 530 } 531 532 // findMappedAddr returns the smallest mapped offAddr that is 533 // >= addr. That is, if addr refers to mapped memory, then it is 534 // returned. If addr is higher than any mapped region, then 535 // it returns maxOffAddr. 536 // 537 // p.mheapLock must be held. 538 func (p *pageAlloc) findMappedAddr(addr offAddr) offAddr { 539 assertLockHeld(p.mheapLock) 540 541 // If we're not in a test, validate first by checking mheap_.arenas. 542 // This is a fast path which is only safe to use outside of testing. 543 ai := arenaIndex(addr.addr()) 544 if p.test || mheap_.arenas[ai.l1()] == nil || mheap_.arenas[ai.l1()][ai.l2()] == nil { 545 vAddr, ok := p.inUse.findAddrGreaterEqual(addr.addr()) 546 if ok { 547 return offAddr{vAddr} 548 } else { 549 // The candidate search address is greater than any 550 // known address, which means we definitely have no 551 // free memory left. 552 return maxOffAddr 553 } 554 } 555 return addr 556 } 557 558 // find searches for the first (address-ordered) contiguous free region of 559 // npages in size and returns a base address for that region. 560 // 561 // It uses p.searchAddr to prune its search and assumes that no palloc chunks 562 // below chunkIndex(p.searchAddr) contain any free memory at all. 563 // 564 // find also computes and returns a candidate p.searchAddr, which may or 565 // may not prune more of the address space than p.searchAddr already does. 566 // This candidate is always a valid p.searchAddr. 567 // 568 // find represents the slow path and the full radix tree search. 569 // 570 // Returns a base address of 0 on failure, in which case the candidate 571 // searchAddr returned is invalid and must be ignored. 572 // 573 // p.mheapLock must be held. 574 func (p *pageAlloc) find(npages uintptr) (uintptr, offAddr) { 575 assertLockHeld(p.mheapLock) 576 577 // Search algorithm. 578 // 579 // This algorithm walks each level l of the radix tree from the root level 580 // to the leaf level. It iterates over at most 1 << levelBits[l] of entries 581 // in a given level in the radix tree, and uses the summary information to 582 // find either: 583 // 1) That a given subtree contains a large enough contiguous region, at 584 // which point it continues iterating on the next level, or 585 // 2) That there are enough contiguous boundary-crossing bits to satisfy 586 // the allocation, at which point it knows exactly where to start 587 // allocating from. 588 // 589 // i tracks the index into the current level l's structure for the 590 // contiguous 1 << levelBits[l] entries we're actually interested in. 591 // 592 // NOTE: Technically this search could allocate a region which crosses 593 // the arenaBaseOffset boundary, which when arenaBaseOffset != 0, is 594 // a discontinuity. However, the only way this could happen is if the 595 // page at the zero address is mapped, and this is impossible on 596 // every system we support where arenaBaseOffset != 0. So, the 597 // discontinuity is already encoded in the fact that the OS will never 598 // map the zero page for us, and this function doesn't try to handle 599 // this case in any way. 600 601 // i is the beginning of the block of entries we're searching at the 602 // current level. 603 i := 0 604 605 // firstFree is the region of address space that we are certain to 606 // find the first free page in the heap. base and bound are the inclusive 607 // bounds of this window, and both are addresses in the linearized, contiguous 608 // view of the address space (with arenaBaseOffset pre-added). At each level, 609 // this window is narrowed as we find the memory region containing the 610 // first free page of memory. To begin with, the range reflects the 611 // full process address space. 612 // 613 // firstFree is updated by calling foundFree each time free space in the 614 // heap is discovered. 615 // 616 // At the end of the search, base.addr() is the best new 617 // searchAddr we could deduce in this search. 618 firstFree := struct { 619 base, bound offAddr 620 }{ 621 base: minOffAddr, 622 bound: maxOffAddr, 623 } 624 // foundFree takes the given address range [addr, addr+size) and 625 // updates firstFree if it is a narrower range. The input range must 626 // either be fully contained within firstFree or not overlap with it 627 // at all. 628 // 629 // This way, we'll record the first summary we find with any free 630 // pages on the root level and narrow that down if we descend into 631 // that summary. But as soon as we need to iterate beyond that summary 632 // in a level to find a large enough range, we'll stop narrowing. 633 foundFree := func(addr offAddr, size uintptr) { 634 if firstFree.base.lessEqual(addr) && addr.add(size-1).lessEqual(firstFree.bound) { 635 // This range fits within the current firstFree window, so narrow 636 // down the firstFree window to the base and bound of this range. 637 firstFree.base = addr 638 firstFree.bound = addr.add(size - 1) 639 } else if !(addr.add(size-1).lessThan(firstFree.base) || firstFree.bound.lessThan(addr)) { 640 // This range only partially overlaps with the firstFree range, 641 // so throw. 642 print("runtime: addr = ", hex(addr.addr()), ", size = ", size, "\n") 643 print("runtime: base = ", hex(firstFree.base.addr()), ", bound = ", hex(firstFree.bound.addr()), "\n") 644 throw("range partially overlaps") 645 } 646 } 647 648 // lastSum is the summary which we saw on the previous level that made us 649 // move on to the next level. Used to print additional information in the 650 // case of a catastrophic failure. 651 // lastSumIdx is that summary's index in the previous level. 652 lastSum := packPallocSum(0, 0, 0) 653 lastSumIdx := -1 654 655 nextLevel: 656 for l := 0; l < len(p.summary); l++ { 657 // For the root level, entriesPerBlock is the whole level. 658 entriesPerBlock := 1 << levelBits[l] 659 logMaxPages := levelLogPages[l] 660 661 // We've moved into a new level, so let's update i to our new 662 // starting index. This is a no-op for level 0. 663 i <<= levelBits[l] 664 665 // Slice out the block of entries we care about. 666 entries := p.summary[l][i : i+entriesPerBlock] 667 668 // Determine j0, the first index we should start iterating from. 669 // The searchAddr may help us eliminate iterations if we followed the 670 // searchAddr on the previous level or we're on the root level, in which 671 // case the searchAddr should be the same as i after levelShift. 672 j0 := 0 673 if searchIdx := offAddrToLevelIndex(l, p.searchAddr); searchIdx&^(entriesPerBlock-1) == i { 674 j0 = searchIdx & (entriesPerBlock - 1) 675 } 676 677 // Run over the level entries looking for 678 // a contiguous run of at least npages either 679 // within an entry or across entries. 680 // 681 // base contains the page index (relative to 682 // the first entry's first page) of the currently 683 // considered run of consecutive pages. 684 // 685 // size contains the size of the currently considered 686 // run of consecutive pages. 687 var base, size uint 688 for j := j0; j < len(entries); j++ { 689 sum := entries[j] 690 if sum == 0 { 691 // A full entry means we broke any streak and 692 // that we should skip it altogether. 693 size = 0 694 continue 695 } 696 697 // We've encountered a non-zero summary which means 698 // free memory, so update firstFree. 699 foundFree(levelIndexToOffAddr(l, i+j), (uintptr(1)<<logMaxPages)*pageSize) 700 701 s := sum.start() 702 if size+s >= uint(npages) { 703 // If size == 0 we don't have a run yet, 704 // which means base isn't valid. So, set 705 // base to the first page in this block. 706 if size == 0 { 707 base = uint(j) << logMaxPages 708 } 709 // We hit npages; we're done! 710 size += s 711 break 712 } 713 if sum.max() >= uint(npages) { 714 // The entry itself contains npages contiguous 715 // free pages, so continue on the next level 716 // to find that run. 717 i += j 718 lastSumIdx = i 719 lastSum = sum 720 continue nextLevel 721 } 722 if size == 0 || s < 1<<logMaxPages { 723 // We either don't have a current run started, or this entry 724 // isn't totally free (meaning we can't continue the current 725 // one), so try to begin a new run by setting size and base 726 // based on sum.end. 727 size = sum.end() 728 base = uint(j+1)<<logMaxPages - size 729 continue 730 } 731 // The entry is completely free, so continue the run. 732 size += 1 << logMaxPages 733 } 734 if size >= uint(npages) { 735 // We found a sufficiently large run of free pages straddling 736 // some boundary, so compute the address and return it. 737 addr := levelIndexToOffAddr(l, i).add(uintptr(base) * pageSize).addr() 738 return addr, p.findMappedAddr(firstFree.base) 739 } 740 if l == 0 { 741 // We're at level zero, so that means we've exhausted our search. 742 return 0, maxSearchAddr() 743 } 744 745 // We're not at level zero, and we exhausted the level we were looking in. 746 // This means that either our calculations were wrong or the level above 747 // lied to us. In either case, dump some useful state and throw. 748 print("runtime: summary[", l-1, "][", lastSumIdx, "] = ", lastSum.start(), ", ", lastSum.max(), ", ", lastSum.end(), "\n") 749 print("runtime: level = ", l, ", npages = ", npages, ", j0 = ", j0, "\n") 750 print("runtime: p.searchAddr = ", hex(p.searchAddr.addr()), ", i = ", i, "\n") 751 print("runtime: levelShift[level] = ", levelShift[l], ", levelBits[level] = ", levelBits[l], "\n") 752 for j := 0; j < len(entries); j++ { 753 sum := entries[j] 754 print("runtime: summary[", l, "][", i+j, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") 755 } 756 throw("bad summary data") 757 } 758 759 // Since we've gotten to this point, that means we haven't found a 760 // sufficiently-sized free region straddling some boundary (chunk or larger). 761 // This means the last summary we inspected must have had a large enough "max" 762 // value, so look inside the chunk to find a suitable run. 763 // 764 // After iterating over all levels, i must contain a chunk index which 765 // is what the final level represents. 766 ci := chunkIdx(i) 767 j, searchIdx := p.chunkOf(ci).find(npages, 0) 768 if j == ^uint(0) { 769 // We couldn't find any space in this chunk despite the summaries telling 770 // us it should be there. There's likely a bug, so dump some state and throw. 771 sum := p.summary[len(p.summary)-1][i] 772 print("runtime: summary[", len(p.summary)-1, "][", i, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") 773 print("runtime: npages = ", npages, "\n") 774 throw("bad summary data") 775 } 776 777 // Compute the address at which the free space starts. 778 addr := chunkBase(ci) + uintptr(j)*pageSize 779 780 // Since we actually searched the chunk, we may have 781 // found an even narrower free window. 782 searchAddr := chunkBase(ci) + uintptr(searchIdx)*pageSize 783 foundFree(offAddr{searchAddr}, chunkBase(ci+1)-searchAddr) 784 return addr, p.findMappedAddr(firstFree.base) 785 } 786 787 // alloc allocates npages worth of memory from the page heap, returning the base 788 // address for the allocation and the amount of scavenged memory in bytes 789 // contained in the region [base address, base address + npages*pageSize). 790 // 791 // Returns a 0 base address on failure, in which case other returned values 792 // should be ignored. 793 // 794 // p.mheapLock must be held. 795 // 796 // Must run on the system stack because p.mheapLock must be held. 797 // 798 //go:systemstack 799 func (p *pageAlloc) alloc(npages uintptr) (addr uintptr, scav uintptr) { 800 assertLockHeld(p.mheapLock) 801 802 // If the searchAddr refers to a region which has a higher address than 803 // any known chunk, then we know we're out of memory. 804 if chunkIndex(p.searchAddr.addr()) >= p.end { 805 return 0, 0 806 } 807 808 // If npages has a chance of fitting in the chunk where the searchAddr is, 809 // search it directly. 810 searchAddr := minOffAddr 811 if pallocChunkPages-chunkPageIndex(p.searchAddr.addr()) >= uint(npages) { 812 // npages is guaranteed to be no greater than pallocChunkPages here. 813 i := chunkIndex(p.searchAddr.addr()) 814 if max := p.summary[len(p.summary)-1][i].max(); max >= uint(npages) { 815 j, searchIdx := p.chunkOf(i).find(npages, chunkPageIndex(p.searchAddr.addr())) 816 if j == ^uint(0) { 817 print("runtime: max = ", max, ", npages = ", npages, "\n") 818 print("runtime: searchIdx = ", chunkPageIndex(p.searchAddr.addr()), ", p.searchAddr = ", hex(p.searchAddr.addr()), "\n") 819 throw("bad summary data") 820 } 821 addr = chunkBase(i) + uintptr(j)*pageSize 822 searchAddr = offAddr{chunkBase(i) + uintptr(searchIdx)*pageSize} 823 goto Found 824 } 825 } 826 // We failed to use a searchAddr for one reason or another, so try 827 // the slow path. 828 addr, searchAddr = p.find(npages) 829 if addr == 0 { 830 if npages == 1 { 831 // We failed to find a single free page, the smallest unit 832 // of allocation. This means we know the heap is completely 833 // exhausted. Otherwise, the heap still might have free 834 // space in it, just not enough contiguous space to 835 // accommodate npages. 836 p.searchAddr = maxSearchAddr() 837 } 838 return 0, 0 839 } 840 Found: 841 // Go ahead and actually mark the bits now that we have an address. 842 scav = p.allocRange(addr, npages) 843 844 // If we found a higher searchAddr, we know that all the 845 // heap memory before that searchAddr in an offset address space is 846 // allocated, so bump p.searchAddr up to the new one. 847 if p.searchAddr.lessThan(searchAddr) { 848 p.searchAddr = searchAddr 849 } 850 return addr, scav 851 } 852 853 // free returns npages worth of memory starting at base back to the page heap. 854 // 855 // p.mheapLock must be held. 856 // 857 // Must run on the system stack because p.mheapLock must be held. 858 // 859 //go:systemstack 860 func (p *pageAlloc) free(base, npages uintptr, scavenged bool) { 861 assertLockHeld(p.mheapLock) 862 863 // If we're freeing pages below the p.searchAddr, update searchAddr. 864 if b := (offAddr{base}); b.lessThan(p.searchAddr) { 865 p.searchAddr = b 866 } 867 limit := base + npages*pageSize - 1 868 if !scavenged { 869 p.scav.index.mark(base, limit+1) 870 } 871 if npages == 1 { 872 // Fast path: we're clearing a single bit, and we know exactly 873 // where it is, so mark it directly. 874 i := chunkIndex(base) 875 p.chunkOf(i).free1(chunkPageIndex(base)) 876 } else { 877 // Slow path: we're clearing more bits so we may need to iterate. 878 sc, ec := chunkIndex(base), chunkIndex(limit) 879 si, ei := chunkPageIndex(base), chunkPageIndex(limit) 880 881 if sc == ec { 882 // The range doesn't cross any chunk boundaries. 883 p.chunkOf(sc).free(si, ei+1-si) 884 } else { 885 // The range crosses at least one chunk boundary. 886 p.chunkOf(sc).free(si, pallocChunkPages-si) 887 for c := sc + 1; c < ec; c++ { 888 p.chunkOf(c).freeAll() 889 } 890 p.chunkOf(ec).free(0, ei+1) 891 } 892 } 893 p.update(base, npages, true, false) 894 } 895 896 const ( 897 pallocSumBytes = unsafe.Sizeof(pallocSum(0)) 898 899 // maxPackedValue is the maximum value that any of the three fields in 900 // the pallocSum may take on. 901 maxPackedValue = 1 << logMaxPackedValue 902 logMaxPackedValue = logPallocChunkPages + (summaryLevels-1)*summaryLevelBits 903 904 freeChunkSum = pallocSum(uint64(pallocChunkPages) | 905 uint64(pallocChunkPages<<logMaxPackedValue) | 906 uint64(pallocChunkPages<<(2*logMaxPackedValue))) 907 ) 908 909 // pallocSum is a packed summary type which packs three numbers: start, max, 910 // and end into a single 8-byte value. Each of these values are a summary of 911 // a bitmap and are thus counts, each of which may have a maximum value of 912 // 2^21 - 1, or all three may be equal to 2^21. The latter case is represented 913 // by just setting the 64th bit. 914 type pallocSum uint64 915 916 // packPallocSum takes a start, max, and end value and produces a pallocSum. 917 func packPallocSum(start, max, end uint) pallocSum { 918 if max == maxPackedValue { 919 return pallocSum(uint64(1 << 63)) 920 } 921 return pallocSum((uint64(start) & (maxPackedValue - 1)) | 922 ((uint64(max) & (maxPackedValue - 1)) << logMaxPackedValue) | 923 ((uint64(end) & (maxPackedValue - 1)) << (2 * logMaxPackedValue))) 924 } 925 926 // start extracts the start value from a packed sum. 927 func (p pallocSum) start() uint { 928 if uint64(p)&uint64(1<<63) != 0 { 929 return maxPackedValue 930 } 931 return uint(uint64(p) & (maxPackedValue - 1)) 932 } 933 934 // max extracts the max value from a packed sum. 935 func (p pallocSum) max() uint { 936 if uint64(p)&uint64(1<<63) != 0 { 937 return maxPackedValue 938 } 939 return uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)) 940 } 941 942 // end extracts the end value from a packed sum. 943 func (p pallocSum) end() uint { 944 if uint64(p)&uint64(1<<63) != 0 { 945 return maxPackedValue 946 } 947 return uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) 948 } 949 950 // unpack unpacks all three values from the summary. 951 func (p pallocSum) unpack() (uint, uint, uint) { 952 if uint64(p)&uint64(1<<63) != 0 { 953 return maxPackedValue, maxPackedValue, maxPackedValue 954 } 955 return uint(uint64(p) & (maxPackedValue - 1)), 956 uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)), 957 uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) 958 } 959 960 // mergeSummaries merges consecutive summaries which may each represent at 961 // most 1 << logMaxPagesPerSum pages each together into one. 962 func mergeSummaries(sums []pallocSum, logMaxPagesPerSum uint) pallocSum { 963 // Merge the summaries in sums into one. 964 // 965 // We do this by keeping a running summary representing the merged 966 // summaries of sums[:i] in start, max, and end. 967 start, max, end := sums[0].unpack() 968 for i := 1; i < len(sums); i++ { 969 // Merge in sums[i]. 970 si, mi, ei := sums[i].unpack() 971 972 // Merge in sums[i].start only if the running summary is 973 // completely free, otherwise this summary's start 974 // plays no role in the combined sum. 975 if start == uint(i)<<logMaxPagesPerSum { 976 start += si 977 } 978 979 // Recompute the max value of the running sum by looking 980 // across the boundary between the running sum and sums[i] 981 // and at the max sums[i], taking the greatest of those two 982 // and the max of the running sum. 983 if end+si > max { 984 max = end + si 985 } 986 if mi > max { 987 max = mi 988 } 989 990 // Merge in end by checking if this new summary is totally 991 // free. If it is, then we want to extend the running sum's 992 // end by the new summary. If not, then we have some alloc'd 993 // pages in there and we just want to take the end value in 994 // sums[i]. 995 if ei == 1<<logMaxPagesPerSum { 996 end += 1 << logMaxPagesPerSum 997 } else { 998 end = ei 999 } 1000 } 1001 return packPallocSum(start, max, end) 1002 }