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