Source file src/runtime/mpagealloc.go
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 "internal/runtime/atomic" 52 "internal/runtime/gc" 53 "unsafe" 54 ) 55 56 const ( 57 // The size of a bitmap chunk, i.e. the amount of bits (that is, pages) to consider 58 // in the bitmap at once. 59 pallocChunkPages = 1 << logPallocChunkPages 60 pallocChunkBytes = pallocChunkPages * pageSize 61 logPallocChunkPages = 9 62 logPallocChunkBytes = logPallocChunkPages + gc.PageShift 63 64 // The number of radix bits for each level. 65 // 66 // The value of 3 is chosen such that the block of summaries we need to scan at 67 // each level fits in 64 bytes (2^3 summaries * 8 bytes per summary), which is 68 // close to the L1 cache line width on many systems. Also, a value of 3 fits 4 tree 69 // levels perfectly into the 21-bit pallocBits summary field at the root level. 70 // 71 // The following equation explains how each of the constants relate: 72 // summaryL0Bits + (summaryLevels-1)*summaryLevelBits + logPallocChunkBytes = heapAddrBits 73 // 74 // summaryLevels is an architecture-dependent value defined in mpagealloc_*.go. 75 summaryLevelBits = 3 76 summaryL0Bits = heapAddrBits - logPallocChunkBytes - (summaryLevels-1)*summaryLevelBits 77 78 // pallocChunksL2Bits is the number of bits of the chunk index number 79 // covered by the second level of the chunks map. 80 // 81 // See (*pageAlloc).chunks for more details. Update the documentation 82 // there should this change. 83 pallocChunksL2Bits = heapAddrBits - logPallocChunkBytes - pallocChunksL1Bits 84 pallocChunksL1Shift = pallocChunksL2Bits 85 86 vmaNamePageAllocIndex = "page alloc index" 87 ) 88 89 // maxSearchAddr returns the maximum searchAddr value, which indicates 90 // that the heap has no free space. 91 // 92 // This function exists just to make it clear that this is the maximum address 93 // for the page allocator's search space. See maxOffAddr for details. 94 // 95 // It's a function (rather than a variable) because it needs to be 96 // usable before package runtime's dynamic initialization is complete. 97 // See #51913 for details. 98 func maxSearchAddr() offAddr { return maxOffAddr } 99 100 // Global chunk index. 101 // 102 // Represents an index into the leaf level of the radix tree. 103 // Similar to arenaIndex, except instead of arenas, it divides the address 104 // space into chunks. 105 type chunkIdx uint 106 107 // chunkIndex returns the global index of the palloc chunk containing the 108 // pointer p. 109 func chunkIndex(p uintptr) chunkIdx { 110 return chunkIdx((p - arenaBaseOffset) / pallocChunkBytes) 111 } 112 113 // chunkBase returns the base address of the palloc chunk at index ci. 114 func chunkBase(ci chunkIdx) uintptr { 115 return uintptr(ci)*pallocChunkBytes + arenaBaseOffset 116 } 117 118 // chunkPageIndex computes the index of the page that contains p, 119 // relative to the chunk which contains p. 120 func chunkPageIndex(p uintptr) uint { 121 return uint(p % pallocChunkBytes / pageSize) 122 } 123 124 // l1 returns the index into the first level of (*pageAlloc).chunks. 125 func (i chunkIdx) l1() uint { 126 if pallocChunksL1Bits == 0 { 127 // Let the compiler optimize this away if there's no 128 // L1 map. 129 return 0 130 } else { 131 return uint(i) >> pallocChunksL1Shift 132 } 133 } 134 135 // l2 returns the index into the second level of (*pageAlloc).chunks. 136 func (i chunkIdx) l2() uint { 137 if pallocChunksL1Bits == 0 { 138 return uint(i) 139 } else { 140 return uint(i) & (1<<pallocChunksL2Bits - 1) 141 } 142 } 143 144 // offAddrToLevelIndex converts an address in the offset address space 145 // to the index into summary[level] containing addr. 146 func offAddrToLevelIndex(level int, addr offAddr) int { 147 return int((addr.a - arenaBaseOffset) >> levelShift[level]) 148 } 149 150 // levelIndexToOffAddr converts an index into summary[level] into 151 // the corresponding address in the offset address space. 152 func levelIndexToOffAddr(level, idx int) offAddr { 153 return offAddr{(uintptr(idx) << levelShift[level]) + arenaBaseOffset} 154 } 155 156 // addrsToSummaryRange converts base and limit pointers into a range 157 // of entries for the given summary level. 158 // 159 // The returned range is inclusive on the lower bound and exclusive on 160 // the upper bound. 161 func addrsToSummaryRange(level int, base, limit uintptr) (lo int, hi int) { 162 // This is slightly more nuanced than just a shift for the exclusive 163 // upper-bound. Note that the exclusive upper bound may be within a 164 // summary at this level, meaning if we just do the obvious computation 165 // hi will end up being an inclusive upper bound. Unfortunately, just 166 // adding 1 to that is too broad since we might be on the very edge 167 // of a summary's max page count boundary for this level 168 // (1 << levelLogPages[level]). So, make limit an inclusive upper bound 169 // then shift, then add 1, so we get an exclusive upper bound at the end. 170 lo = int((base - arenaBaseOffset) >> levelShift[level]) 171 hi = int(((limit-1)-arenaBaseOffset)>>levelShift[level]) + 1 172 return 173 } 174 175 // blockAlignSummaryRange aligns indices into the given level to that 176 // level's block width (1 << levelBits[level]). It assumes lo is inclusive 177 // and hi is exclusive, and so aligns them down and up respectively. 178 func blockAlignSummaryRange(level int, lo, hi int) (int, int) { 179 e := uintptr(1) << levelBits[level] 180 return int(alignDown(uintptr(lo), e)), int(alignUp(uintptr(hi), e)) 181 } 182 183 type pageAlloc struct { 184 // Radix tree of summaries. 185 // 186 // Each slice's cap represents the whole memory reservation. 187 // Each slice's len reflects the allocator's maximum known 188 // mapped heap address for that level. 189 // 190 // The backing store of each summary level is reserved in init 191 // and may or may not be committed in grow (small address spaces 192 // may commit all the memory in init). 193 // 194 // The purpose of keeping len <= cap is to enforce bounds checks 195 // on the top end of the slice so that instead of an unknown 196 // runtime segmentation fault, we get a much friendlier out-of-bounds 197 // error. 198 // 199 // To iterate over a summary level, use inUse to determine which ranges 200 // are currently available. Otherwise one might try to access 201 // memory which is only Reserved which may result in a hard fault. 202 // 203 // We may still get segmentation faults < len since some of that 204 // memory may not be committed yet. 205 summary [summaryLevels][]pallocSum 206 207 // chunks is a slice of bitmap chunks. 208 // 209 // The total size of chunks is quite large on most 64-bit platforms 210 // (O(GiB) or more) if flattened, so rather than making one large mapping 211 // (which has problems on some platforms, even when PROT_NONE) we use a 212 // two-level sparse array approach similar to the arena index in mheap. 213 // 214 // To find the chunk containing a memory address `a`, do: 215 // chunkOf(chunkIndex(a)) 216 // 217 // Below is a table describing the configuration for chunks for various 218 // heapAddrBits supported by the runtime. 219 // 220 // heapAddrBits | L1 Bits | L2 Bits | L2 Entry Size 221 // ------------------------------------------------ 222 // 32 | 0 | 10 | 128 KiB 223 // 33 (iOS) | 0 | 11 | 256 KiB 224 // 48 | 13 | 13 | 1 MiB 225 // 226 // There's no reason to use the L1 part of chunks on 32-bit, the 227 // address space is small so the L2 is small. For platforms with a 228 // 48-bit address space, we pick the L1 such that the L2 is 1 MiB 229 // in size, which is a good balance between low granularity without 230 // making the impact on BSS too high (note the L1 is stored directly 231 // in pageAlloc). 232 // 233 // To iterate over the bitmap, use inUse to determine which ranges 234 // are currently available. Otherwise one might iterate over unused 235 // ranges. 236 // 237 // Protected by mheapLock. 238 // 239 // TODO(mknyszek): Consider changing the definition of the bitmap 240 // such that 1 means free and 0 means in-use so that summaries and 241 // the bitmaps align better on zero-values. 242 chunks [1 << pallocChunksL1Bits]*[1 << pallocChunksL2Bits]pallocData 243 244 // The address to start an allocation search with. It must never 245 // point to any memory that is not contained in inUse, i.e. 246 // inUse.contains(searchAddr.addr()) must always be true. The one 247 // exception to this rule is that it may take on the value of 248 // maxOffAddr to indicate that the heap is exhausted. 249 // 250 // We guarantee that all valid heap addresses below this value 251 // are allocated and not worth searching. 252 searchAddr offAddr 253 254 // start and end represent the chunk indices 255 // which pageAlloc knows about. It assumes 256 // chunks in the range [start, end) are 257 // currently ready to use. 258 start, end chunkIdx 259 260 // inUse is a slice of ranges of address space which are 261 // known by the page allocator to be currently in-use (passed 262 // to grow). 263 // 264 // We care much more about having a contiguous heap in these cases 265 // and take additional measures to ensure that, so in nearly all 266 // cases this should have just 1 element. 267 // 268 // All access is protected by the mheapLock. 269 inUse addrRanges 270 271 // scav stores the scavenger state. 272 scav struct { 273 // index is an efficient index of chunks that have pages available to 274 // scavenge. 275 index scavengeIndex 276 277 // releasedBg is the amount of memory released in the background this 278 // scavenge cycle. 279 releasedBg atomic.Uintptr 280 281 // releasedEager is the amount of memory released eagerly this scavenge 282 // cycle. 283 releasedEager atomic.Uintptr 284 } 285 286 // mheap_.lock. This level of indirection makes it possible 287 // to test pageAlloc independently of the runtime allocator. 288 mheapLock *mutex 289 290 // sysStat is the runtime memstat to update when new system 291 // memory is committed by the pageAlloc for allocation metadata. 292 sysStat *sysMemStat 293 294 // summaryMappedReady is the number of bytes mapped in the Ready state 295 // in the summary structure. Used only for testing currently. 296 // 297 // Protected by mheapLock. 298 summaryMappedReady uintptr 299 300 // chunkHugePages indicates whether page bitmap chunks should be backed 301 // by huge pages. 302 chunkHugePages bool 303 304 // Whether or not this struct is being used in tests. 305 test bool 306 } 307 308 func (p *pageAlloc) init(mheapLock *mutex, sysStat *sysMemStat, test bool) { 309 if levelLogPages[0] > logMaxPackedValue { 310 // We can't represent 1<<levelLogPages[0] pages, the maximum number 311 // of pages we need to represent at the root level, in a summary, which 312 // is a big problem. Throw. 313 print("runtime: root level max pages = ", 1<<levelLogPages[0], "\n") 314 print("runtime: summary max pages = ", maxPackedValue, "\n") 315 throw("root level max pages doesn't fit in summary") 316 } 317 p.sysStat = sysStat 318 319 // Initialize p.inUse. 320 p.inUse.init(sysStat) 321 322 // System-dependent initialization. 323 p.sysInit(test) 324 325 // Start with the searchAddr in a state indicating there's no free memory. 326 p.searchAddr = maxSearchAddr() 327 328 // Set the mheapLock. 329 p.mheapLock = mheapLock 330 331 // Initialize the scavenge index. 332 p.summaryMappedReady += p.scav.index.init(test, sysStat) 333 334 // Set if we're in a test. 335 p.test = test 336 } 337 338 // tryChunkOf returns the bitmap data for the given chunk. 339 // 340 // Returns nil if the chunk data has not been mapped. 341 func (p *pageAlloc) tryChunkOf(ci chunkIdx) *pallocData { 342 l2 := p.chunks[ci.l1()] 343 if l2 == nil { 344 return nil 345 } 346 return &l2[ci.l2()] 347 } 348 349 // chunkOf returns the chunk at the given chunk index. 350 // 351 // The chunk index must be valid or this method may throw. 352 func (p *pageAlloc) chunkOf(ci chunkIdx) *pallocData { 353 return &p.chunks[ci.l1()][ci.l2()] 354 } 355 356 // grow sets up the metadata for the address range [base, base+size). 357 // It may allocate metadata, in which case *p.sysStat will be updated. 358 // 359 // p.mheapLock must be held. 360 func (p *pageAlloc) grow(base, size uintptr) { 361 assertLockHeld(p.mheapLock) 362 363 // Round up to chunks, since we can't deal with increments smaller 364 // than chunks. Also, sysGrow expects aligned values. 365 limit := alignUp(base+size, pallocChunkBytes) 366 base = alignDown(base, pallocChunkBytes) 367 368 // Grow the summary levels in a system-dependent manner. 369 // We just update a bunch of additional metadata here. 370 p.sysGrow(base, limit) 371 372 // Grow the scavenge index. 373 p.summaryMappedReady += p.scav.index.grow(base, limit, p.sysStat) 374 375 // Update p.start and p.end. 376 // If no growth happened yet, start == 0. This is generally 377 // safe since the zero page is unmapped. 378 firstGrowth := p.start == 0 379 start, end := chunkIndex(base), chunkIndex(limit) 380 if firstGrowth || start < p.start { 381 p.start = start 382 } 383 if end > p.end { 384 p.end = end 385 } 386 // Note that [base, limit) will never overlap with any existing 387 // range inUse because grow only ever adds never-used memory 388 // regions to the page allocator. 389 p.inUse.add(makeAddrRange(base, limit)) 390 391 // A grow operation is a lot like a free operation, so if our 392 // chunk ends up below p.searchAddr, update p.searchAddr to the 393 // new address, just like in free. 394 if b := (offAddr{base}); b.lessThan(p.searchAddr) { 395 p.searchAddr = b 396 } 397 398 // Add entries into chunks, which is sparse, if needed. Then, 399 // initialize the bitmap. 400 // 401 // Newly-grown memory is always considered scavenged. 402 // Set all the bits in the scavenged bitmaps high. 403 for c := chunkIndex(base); c < chunkIndex(limit); c++ { 404 if p.chunks[c.l1()] == nil { 405 // Create the necessary l2 entry. 406 const l2Size = unsafe.Sizeof(*p.chunks[0]) 407 r := sysAlloc(l2Size, p.sysStat, vmaNamePageAllocIndex) 408 if r == nil { 409 throw("pageAlloc: out of memory") 410 } 411 if !p.test { 412 // Make the chunk mapping eligible or ineligible 413 // for huge pages, depending on what our current 414 // state is. 415 if p.chunkHugePages { 416 sysHugePage(r, l2Size) 417 } else { 418 sysNoHugePage(r, l2Size) 419 } 420 } 421 // Store the new chunk block but avoid a write barrier. 422 // grow is used in call chains that disallow write barriers. 423 *(*uintptr)(unsafe.Pointer(&p.chunks[c.l1()])) = uintptr(r) 424 } 425 p.chunkOf(c).scavenged.setRange(0, pallocChunkPages) 426 } 427 428 // Update summaries accordingly. The grow acts like a free, so 429 // we need to ensure this newly-free memory is visible in the 430 // summaries. 431 p.update(base, size/pageSize, true, false) 432 } 433 434 // enableChunkHugePages enables huge pages for the chunk bitmap mappings (disabled by default). 435 // 436 // This function is idempotent. 437 // 438 // A note on latency: for sufficiently small heaps (<10s of GiB) this function will take constant 439 // time, but may take time proportional to the size of the mapped heap beyond that. 440 // 441 // The heap lock must not be held over this operation, since it will briefly acquire 442 // the heap lock. 443 // 444 // Must be called on the system stack because it acquires the heap lock. 445 // 446 //go:systemstack 447 func (p *pageAlloc) enableChunkHugePages() { 448 // Grab the heap lock to turn on huge pages for new chunks and clone the current 449 // heap address space ranges. 450 // 451 // After the lock is released, we can be sure that bitmaps for any new chunks may 452 // be backed with huge pages, and we have the address space for the rest of the 453 // chunks. At the end of this function, all chunk metadata should be backed by huge 454 // pages. 455 lock(&mheap_.lock) 456 if p.chunkHugePages { 457 unlock(&mheap_.lock) 458 return 459 } 460 p.chunkHugePages = true 461 var inUse addrRanges 462 inUse.sysStat = p.sysStat 463 p.inUse.cloneInto(&inUse) 464 unlock(&mheap_.lock) 465 466 // This might seem like a lot of work, but all these loops are for generality. 467 // 468 // For a 1 GiB contiguous heap, a 48-bit address space, 13 L1 bits, a palloc chunk size 469 // of 4 MiB, and adherence to the default set of heap address hints, this will result in 470 // exactly 1 call to sysHugePage. 471 for _, r := range p.inUse.ranges { 472 for i := chunkIndex(r.base.addr()).l1(); i < chunkIndex(r.limit.addr()-1).l1(); i++ { 473 // N.B. We can assume that p.chunks[i] is non-nil and in a mapped part of p.chunks 474 // because it's derived from inUse, which never shrinks. 475 sysHugePage(unsafe.Pointer(p.chunks[i]), unsafe.Sizeof(*p.chunks[0])) 476 } 477 } 478 } 479 480 // update updates heap metadata. It must be called each time the bitmap 481 // is updated. 482 // 483 // If contig is true, update does some optimizations assuming that there was 484 // a contiguous allocation or free between addr and addr+npages. alloc indicates 485 // whether the operation performed was an allocation or a free. 486 // 487 // p.mheapLock must be held. 488 func (p *pageAlloc) update(base, npages uintptr, contig, alloc bool) { 489 assertLockHeld(p.mheapLock) 490 491 // base, limit, start, and end are inclusive. 492 limit := base + npages*pageSize - 1 493 sc, ec := chunkIndex(base), chunkIndex(limit) 494 495 // Handle updating the lowest level first. 496 if sc == ec { 497 // Fast path: the allocation doesn't span more than one chunk, 498 // so update this one and if the summary didn't change, return. 499 x := p.summary[len(p.summary)-1][sc] 500 y := p.chunkOf(sc).summarize() 501 if x == y { 502 return 503 } 504 p.summary[len(p.summary)-1][sc] = y 505 } else if contig { 506 // Slow contiguous path: the allocation spans more than one chunk 507 // and at least one summary is guaranteed to change. 508 summary := p.summary[len(p.summary)-1] 509 510 // Update the summary for chunk sc. 511 summary[sc] = p.chunkOf(sc).summarize() 512 513 // Update the summaries for chunks in between, which are 514 // either totally allocated or freed. 515 whole := p.summary[len(p.summary)-1][sc+1 : ec] 516 if alloc { 517 clear(whole) 518 } else { 519 for i := range whole { 520 whole[i] = freeChunkSum 521 } 522 } 523 524 // Update the summary for chunk ec. 525 summary[ec] = p.chunkOf(ec).summarize() 526 } else { 527 // Slow general path: the allocation spans more than one chunk 528 // and at least one summary is guaranteed to change. 529 // 530 // We can't assume a contiguous allocation happened, so walk over 531 // every chunk in the range and manually recompute the summary. 532 summary := p.summary[len(p.summary)-1] 533 for c := sc; c <= ec; c++ { 534 summary[c] = p.chunkOf(c).summarize() 535 } 536 } 537 538 // Walk up the radix tree and update the summaries appropriately. 539 changed := true 540 for l := len(p.summary) - 2; l >= 0 && changed; l-- { 541 // Update summaries at level l from summaries at level l+1. 542 changed = false 543 544 // "Constants" for the previous level which we 545 // need to compute the summary from that level. 546 logEntriesPerBlock := levelBits[l+1] 547 logMaxPages := levelLogPages[l+1] 548 549 // lo and hi describe all the parts of the level we need to look at. 550 lo, hi := addrsToSummaryRange(l, base, limit+1) 551 552 // Iterate over each block, updating the corresponding summary in the less-granular level. 553 for i := lo; i < hi; i++ { 554 children := p.summary[l+1][i<<logEntriesPerBlock : (i+1)<<logEntriesPerBlock] 555 sum := mergeSummaries(children, logMaxPages) 556 old := p.summary[l][i] 557 if old != sum { 558 changed = true 559 p.summary[l][i] = sum 560 } 561 } 562 } 563 } 564 565 // allocRange marks the range of memory [base, base+npages*pageSize) as 566 // allocated. It also updates the summaries to reflect the newly-updated 567 // bitmap. 568 // 569 // Returns the amount of scavenged memory in bytes present in the 570 // allocated range. 571 // 572 // p.mheapLock must be held. 573 func (p *pageAlloc) allocRange(base, npages uintptr) uintptr { 574 assertLockHeld(p.mheapLock) 575 576 limit := base + npages*pageSize - 1 577 sc, ec := chunkIndex(base), chunkIndex(limit) 578 si, ei := chunkPageIndex(base), chunkPageIndex(limit) 579 580 scav := uint(0) 581 if sc == ec { 582 // The range doesn't cross any chunk boundaries. 583 chunk := p.chunkOf(sc) 584 scav += chunk.scavenged.popcntRange(si, ei+1-si) 585 chunk.allocRange(si, ei+1-si) 586 p.scav.index.alloc(sc, ei+1-si) 587 } else { 588 // The range crosses at least one chunk boundary. 589 chunk := p.chunkOf(sc) 590 scav += chunk.scavenged.popcntRange(si, pallocChunkPages-si) 591 chunk.allocRange(si, pallocChunkPages-si) 592 p.scav.index.alloc(sc, pallocChunkPages-si) 593 for c := sc + 1; c < ec; c++ { 594 chunk := p.chunkOf(c) 595 scav += chunk.scavenged.popcntRange(0, pallocChunkPages) 596 chunk.allocAll() 597 p.scav.index.alloc(c, pallocChunkPages) 598 } 599 chunk = p.chunkOf(ec) 600 scav += chunk.scavenged.popcntRange(0, ei+1) 601 chunk.allocRange(0, ei+1) 602 p.scav.index.alloc(ec, ei+1) 603 } 604 p.update(base, npages, true, true) 605 return uintptr(scav) * pageSize 606 } 607 608 // findMappedAddr returns the smallest mapped offAddr that is 609 // >= addr. That is, if addr refers to mapped memory, then it is 610 // returned. If addr is higher than any mapped region, then 611 // it returns maxOffAddr. 612 // 613 // p.mheapLock must be held. 614 func (p *pageAlloc) findMappedAddr(addr offAddr) offAddr { 615 assertLockHeld(p.mheapLock) 616 617 // If we're not in a test, validate first by checking mheap_.arenas. 618 // This is a fast path which is only safe to use outside of testing. 619 ai := arenaIndex(addr.addr()) 620 if p.test || mheap_.arenas[ai.l1()] == nil || mheap_.arenas[ai.l1()][ai.l2()] == nil { 621 vAddr, ok := p.inUse.findAddrGreaterEqual(addr.addr()) 622 if ok { 623 return offAddr{vAddr} 624 } else { 625 // The candidate search address is greater than any 626 // known address, which means we definitely have no 627 // free memory left. 628 return maxOffAddr 629 } 630 } 631 return addr 632 } 633 634 // find searches for the first (address-ordered) contiguous free region of 635 // npages in size and returns a base address for that region. 636 // 637 // It uses p.searchAddr to prune its search and assumes that no palloc chunks 638 // below chunkIndex(p.searchAddr) contain any free memory at all. 639 // 640 // find also computes and returns a candidate p.searchAddr, which may or 641 // may not prune more of the address space than p.searchAddr already does. 642 // This candidate is always a valid p.searchAddr. 643 // 644 // find represents the slow path and the full radix tree search. 645 // 646 // Returns a base address of 0 on failure, in which case the candidate 647 // searchAddr returned is invalid and must be ignored. 648 // 649 // p.mheapLock must be held. 650 func (p *pageAlloc) find(npages uintptr) (uintptr, offAddr) { 651 assertLockHeld(p.mheapLock) 652 653 // Search algorithm. 654 // 655 // This algorithm walks each level l of the radix tree from the root level 656 // to the leaf level. It iterates over at most 1 << levelBits[l] of entries 657 // in a given level in the radix tree, and uses the summary information to 658 // find either: 659 // 1) That a given subtree contains a large enough contiguous region, at 660 // which point it continues iterating on the next level, or 661 // 2) That there are enough contiguous boundary-crossing bits to satisfy 662 // the allocation, at which point it knows exactly where to start 663 // allocating from. 664 // 665 // i tracks the index into the current level l's structure for the 666 // contiguous 1 << levelBits[l] entries we're actually interested in. 667 // 668 // NOTE: Technically this search could allocate a region which crosses 669 // the arenaBaseOffset boundary, which when arenaBaseOffset != 0, is 670 // a discontinuity. However, the only way this could happen is if the 671 // page at the zero address is mapped, and this is impossible on 672 // every system we support where arenaBaseOffset != 0. So, the 673 // discontinuity is already encoded in the fact that the OS will never 674 // map the zero page for us, and this function doesn't try to handle 675 // this case in any way. 676 677 // i is the beginning of the block of entries we're searching at the 678 // current level. 679 i := 0 680 681 // firstFree is the region of address space that we are certain to 682 // find the first free page in the heap. base and bound are the inclusive 683 // bounds of this window, and both are addresses in the linearized, contiguous 684 // view of the address space (with arenaBaseOffset pre-added). At each level, 685 // this window is narrowed as we find the memory region containing the 686 // first free page of memory. To begin with, the range reflects the 687 // full process address space. 688 // 689 // firstFree is updated by calling foundFree each time free space in the 690 // heap is discovered. 691 // 692 // At the end of the search, base.addr() is the best new 693 // searchAddr we could deduce in this search. 694 firstFree := struct { 695 base, bound offAddr 696 }{ 697 base: minOffAddr, 698 bound: maxOffAddr, 699 } 700 // foundFree takes the given address range [addr, addr+size) and 701 // updates firstFree if it is a narrower range. The input range must 702 // either be fully contained within firstFree or not overlap with it 703 // at all. 704 // 705 // This way, we'll record the first summary we find with any free 706 // pages on the root level and narrow that down if we descend into 707 // that summary. But as soon as we need to iterate beyond that summary 708 // in a level to find a large enough range, we'll stop narrowing. 709 foundFree := func(addr offAddr, size uintptr) { 710 if firstFree.base.lessEqual(addr) && addr.add(size-1).lessEqual(firstFree.bound) { 711 // This range fits within the current firstFree window, so narrow 712 // down the firstFree window to the base and bound of this range. 713 firstFree.base = addr 714 firstFree.bound = addr.add(size - 1) 715 } else if !(addr.add(size-1).lessThan(firstFree.base) || firstFree.bound.lessThan(addr)) { 716 // This range only partially overlaps with the firstFree range, 717 // so throw. 718 print("runtime: addr = ", hex(addr.addr()), ", size = ", size, "\n") 719 print("runtime: base = ", hex(firstFree.base.addr()), ", bound = ", hex(firstFree.bound.addr()), "\n") 720 throw("range partially overlaps") 721 } 722 } 723 724 // lastSum is the summary which we saw on the previous level that made us 725 // move on to the next level. Used to print additional information in the 726 // case of a catastrophic failure. 727 // lastSumIdx is that summary's index in the previous level. 728 lastSum := packPallocSum(0, 0, 0) 729 lastSumIdx := -1 730 731 nextLevel: 732 for l := 0; l < len(p.summary); l++ { 733 // For the root level, entriesPerBlock is the whole level. 734 entriesPerBlock := 1 << levelBits[l] 735 logMaxPages := levelLogPages[l] 736 737 // We've moved into a new level, so let's update i to our new 738 // starting index. This is a no-op for level 0. 739 i <<= levelBits[l] 740 741 // Slice out the block of entries we care about. 742 entries := p.summary[l][i : i+entriesPerBlock] 743 744 // Determine j0, the first index we should start iterating from. 745 // The searchAddr may help us eliminate iterations if we followed the 746 // searchAddr on the previous level or we're on the root level, in which 747 // case the searchAddr should be the same as i after levelShift. 748 j0 := 0 749 if searchIdx := offAddrToLevelIndex(l, p.searchAddr); searchIdx&^(entriesPerBlock-1) == i { 750 j0 = searchIdx & (entriesPerBlock - 1) 751 } 752 753 // Run over the level entries looking for 754 // a contiguous run of at least npages either 755 // within an entry or across entries. 756 // 757 // base contains the page index (relative to 758 // the first entry's first page) of the currently 759 // considered run of consecutive pages. 760 // 761 // size contains the size of the currently considered 762 // run of consecutive pages. 763 var base, size uint 764 for j := j0; j < len(entries); j++ { 765 sum := entries[j] 766 if sum == 0 { 767 // A full entry means we broke any streak and 768 // that we should skip it altogether. 769 size = 0 770 continue 771 } 772 773 // We've encountered a non-zero summary which means 774 // free memory, so update firstFree. 775 foundFree(levelIndexToOffAddr(l, i+j), (uintptr(1)<<logMaxPages)*pageSize) 776 777 s := sum.start() 778 if size+s >= uint(npages) { 779 // If size == 0 we don't have a run yet, 780 // which means base isn't valid. So, set 781 // base to the first page in this block. 782 if size == 0 { 783 base = uint(j) << logMaxPages 784 } 785 // We hit npages; we're done! 786 size += s 787 break 788 } 789 if sum.max() >= uint(npages) { 790 // The entry itself contains npages contiguous 791 // free pages, so continue on the next level 792 // to find that run. 793 i += j 794 lastSumIdx = i 795 lastSum = sum 796 continue nextLevel 797 } 798 if size == 0 || s < 1<<logMaxPages { 799 // We either don't have a current run started, or this entry 800 // isn't totally free (meaning we can't continue the current 801 // one), so try to begin a new run by setting size and base 802 // based on sum.end. 803 size = sum.end() 804 base = uint(j+1)<<logMaxPages - size 805 continue 806 } 807 // The entry is completely free, so continue the run. 808 size += 1 << logMaxPages 809 } 810 if size >= uint(npages) { 811 // We found a sufficiently large run of free pages straddling 812 // some boundary, so compute the address and return it. 813 addr := levelIndexToOffAddr(l, i).add(uintptr(base) * pageSize).addr() 814 return addr, p.findMappedAddr(firstFree.base) 815 } 816 if l == 0 { 817 // We're at level zero, so that means we've exhausted our search. 818 return 0, maxSearchAddr() 819 } 820 821 // We're not at level zero, and we exhausted the level we were looking in. 822 // This means that either our calculations were wrong or the level above 823 // lied to us. In either case, dump some useful state and throw. 824 print("runtime: summary[", l-1, "][", lastSumIdx, "] = ", lastSum.start(), ", ", lastSum.max(), ", ", lastSum.end(), "\n") 825 print("runtime: level = ", l, ", npages = ", npages, ", j0 = ", j0, "\n") 826 print("runtime: p.searchAddr = ", hex(p.searchAddr.addr()), ", i = ", i, "\n") 827 print("runtime: levelShift[level] = ", levelShift[l], ", levelBits[level] = ", levelBits[l], "\n") 828 for j := 0; j < len(entries); j++ { 829 sum := entries[j] 830 print("runtime: summary[", l, "][", i+j, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") 831 } 832 throw("bad summary data") 833 } 834 835 // Since we've gotten to this point, that means we haven't found a 836 // sufficiently-sized free region straddling some boundary (chunk or larger). 837 // This means the last summary we inspected must have had a large enough "max" 838 // value, so look inside the chunk to find a suitable run. 839 // 840 // After iterating over all levels, i must contain a chunk index which 841 // is what the final level represents. 842 ci := chunkIdx(i) 843 j, searchIdx := p.chunkOf(ci).find(npages, 0) 844 if j == ^uint(0) { 845 // We couldn't find any space in this chunk despite the summaries telling 846 // us it should be there. There's likely a bug, so dump some state and throw. 847 sum := p.summary[len(p.summary)-1][i] 848 print("runtime: summary[", len(p.summary)-1, "][", i, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") 849 print("runtime: npages = ", npages, "\n") 850 throw("bad summary data") 851 } 852 853 // Compute the address at which the free space starts. 854 addr := chunkBase(ci) + uintptr(j)*pageSize 855 856 // Since we actually searched the chunk, we may have 857 // found an even narrower free window. 858 searchAddr := chunkBase(ci) + uintptr(searchIdx)*pageSize 859 foundFree(offAddr{searchAddr}, chunkBase(ci+1)-searchAddr) 860 return addr, p.findMappedAddr(firstFree.base) 861 } 862 863 // alloc allocates npages worth of memory from the page heap, returning the base 864 // address for the allocation and the amount of scavenged memory in bytes 865 // contained in the region [base address, base address + npages*pageSize). 866 // 867 // Returns a 0 base address on failure, in which case other returned values 868 // should be ignored. 869 // 870 // p.mheapLock must be held. 871 // 872 // Must run on the system stack because p.mheapLock must be held. 873 // 874 //go:systemstack 875 func (p *pageAlloc) alloc(npages uintptr) (addr uintptr, scav uintptr) { 876 assertLockHeld(p.mheapLock) 877 878 // If the searchAddr refers to a region which has a higher address than 879 // any known chunk, then we know we're out of memory. 880 if chunkIndex(p.searchAddr.addr()) >= p.end { 881 return 0, 0 882 } 883 884 // If npages has a chance of fitting in the chunk where the searchAddr is, 885 // search it directly. 886 searchAddr := minOffAddr 887 if pallocChunkPages-chunkPageIndex(p.searchAddr.addr()) >= uint(npages) { 888 // npages is guaranteed to be no greater than pallocChunkPages here. 889 i := chunkIndex(p.searchAddr.addr()) 890 if max := p.summary[len(p.summary)-1][i].max(); max >= uint(npages) { 891 j, searchIdx := p.chunkOf(i).find(npages, chunkPageIndex(p.searchAddr.addr())) 892 if j == ^uint(0) { 893 print("runtime: max = ", max, ", npages = ", npages, "\n") 894 print("runtime: searchIdx = ", chunkPageIndex(p.searchAddr.addr()), ", p.searchAddr = ", hex(p.searchAddr.addr()), "\n") 895 throw("bad summary data") 896 } 897 addr = chunkBase(i) + uintptr(j)*pageSize 898 searchAddr = offAddr{chunkBase(i) + uintptr(searchIdx)*pageSize} 899 goto Found 900 } 901 } 902 // We failed to use a searchAddr for one reason or another, so try 903 // the slow path. 904 addr, searchAddr = p.find(npages) 905 if addr == 0 { 906 if npages == 1 { 907 // We failed to find a single free page, the smallest unit 908 // of allocation. This means we know the heap is completely 909 // exhausted. Otherwise, the heap still might have free 910 // space in it, just not enough contiguous space to 911 // accommodate npages. 912 p.searchAddr = maxSearchAddr() 913 } 914 return 0, 0 915 } 916 Found: 917 // Go ahead and actually mark the bits now that we have an address. 918 scav = p.allocRange(addr, npages) 919 920 // If we found a higher searchAddr, we know that all the 921 // heap memory before that searchAddr in an offset address space is 922 // allocated, so bump p.searchAddr up to the new one. 923 if p.searchAddr.lessThan(searchAddr) { 924 p.searchAddr = searchAddr 925 } 926 return addr, scav 927 } 928 929 // free returns npages worth of memory starting at base back to the page heap. 930 // 931 // p.mheapLock must be held. 932 // 933 // Must run on the system stack because p.mheapLock must be held. 934 // 935 //go:systemstack 936 func (p *pageAlloc) free(base, npages uintptr) { 937 assertLockHeld(p.mheapLock) 938 939 // If we're freeing pages below the p.searchAddr, update searchAddr. 940 if b := (offAddr{base}); b.lessThan(p.searchAddr) { 941 p.searchAddr = b 942 } 943 limit := base + npages*pageSize - 1 944 if npages == 1 { 945 // Fast path: we're clearing a single bit, and we know exactly 946 // where it is, so mark it directly. 947 i := chunkIndex(base) 948 pi := chunkPageIndex(base) 949 p.chunkOf(i).free1(pi) 950 p.scav.index.free(i, pi, 1) 951 } else { 952 // Slow path: we're clearing more bits so we may need to iterate. 953 sc, ec := chunkIndex(base), chunkIndex(limit) 954 si, ei := chunkPageIndex(base), chunkPageIndex(limit) 955 956 if sc == ec { 957 // The range doesn't cross any chunk boundaries. 958 p.chunkOf(sc).free(si, ei+1-si) 959 p.scav.index.free(sc, si, ei+1-si) 960 } else { 961 // The range crosses at least one chunk boundary. 962 p.chunkOf(sc).free(si, pallocChunkPages-si) 963 p.scav.index.free(sc, si, pallocChunkPages-si) 964 for c := sc + 1; c < ec; c++ { 965 p.chunkOf(c).freeAll() 966 p.scav.index.free(c, 0, pallocChunkPages) 967 } 968 p.chunkOf(ec).free(0, ei+1) 969 p.scav.index.free(ec, 0, ei+1) 970 } 971 } 972 p.update(base, npages, true, false) 973 } 974 975 const ( 976 pallocSumBytes = unsafe.Sizeof(pallocSum(0)) 977 978 // maxPackedValue is the maximum value that any of the three fields in 979 // the pallocSum may take on. 980 maxPackedValue = 1 << logMaxPackedValue 981 logMaxPackedValue = logPallocChunkPages + (summaryLevels-1)*summaryLevelBits 982 983 freeChunkSum = pallocSum(uint64(pallocChunkPages) | 984 uint64(pallocChunkPages<<logMaxPackedValue) | 985 uint64(pallocChunkPages<<(2*logMaxPackedValue))) 986 ) 987 988 // pallocSum is a packed summary type which packs three numbers: start, max, 989 // and end into a single 8-byte value. Each of these values are a summary of 990 // a bitmap and are thus counts, each of which may have a maximum value of 991 // 2^21 - 1, or all three may be equal to 2^21. The latter case is represented 992 // by just setting the 64th bit. 993 type pallocSum uint64 994 995 // packPallocSum takes a start, max, and end value and produces a pallocSum. 996 func packPallocSum(start, max, end uint) pallocSum { 997 if max == maxPackedValue { 998 return pallocSum(uint64(1 << 63)) 999 } 1000 return pallocSum((uint64(start) & (maxPackedValue - 1)) | 1001 ((uint64(max) & (maxPackedValue - 1)) << logMaxPackedValue) | 1002 ((uint64(end) & (maxPackedValue - 1)) << (2 * logMaxPackedValue))) 1003 } 1004 1005 // start extracts the start value from a packed sum. 1006 func (p pallocSum) start() uint { 1007 if uint64(p)&uint64(1<<63) != 0 { 1008 return maxPackedValue 1009 } 1010 return uint(uint64(p) & (maxPackedValue - 1)) 1011 } 1012 1013 // max extracts the max value from a packed sum. 1014 func (p pallocSum) max() uint { 1015 if uint64(p)&uint64(1<<63) != 0 { 1016 return maxPackedValue 1017 } 1018 return uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)) 1019 } 1020 1021 // end extracts the end value from a packed sum. 1022 func (p pallocSum) end() uint { 1023 if uint64(p)&uint64(1<<63) != 0 { 1024 return maxPackedValue 1025 } 1026 return uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) 1027 } 1028 1029 // unpack unpacks all three values from the summary. 1030 func (p pallocSum) unpack() (uint, uint, uint) { 1031 if uint64(p)&uint64(1<<63) != 0 { 1032 return maxPackedValue, maxPackedValue, maxPackedValue 1033 } 1034 return uint(uint64(p) & (maxPackedValue - 1)), 1035 uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)), 1036 uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) 1037 } 1038 1039 // mergeSummaries merges consecutive summaries which may each represent at 1040 // most 1 << logMaxPagesPerSum pages each together into one. 1041 func mergeSummaries(sums []pallocSum, logMaxPagesPerSum uint) pallocSum { 1042 // Merge the summaries in sums into one. 1043 // 1044 // We do this by keeping a running summary representing the merged 1045 // summaries of sums[:i] in start, most, and end. 1046 start, most, end := sums[0].unpack() 1047 for i := 1; i < len(sums); i++ { 1048 // Merge in sums[i]. 1049 si, mi, ei := sums[i].unpack() 1050 1051 // Merge in sums[i].start only if the running summary is 1052 // completely free, otherwise this summary's start 1053 // plays no role in the combined sum. 1054 if start == uint(i)<<logMaxPagesPerSum { 1055 start += si 1056 } 1057 1058 // Recompute the max value of the running sum by looking 1059 // across the boundary between the running sum and sums[i] 1060 // and at the max sums[i], taking the greatest of those two 1061 // and the max of the running sum. 1062 most = max(most, end+si, mi) 1063 1064 // Merge in end by checking if this new summary is totally 1065 // free. If it is, then we want to extend the running sum's 1066 // end by the new summary. If not, then we have some alloc'd 1067 // pages in there and we just want to take the end value in 1068 // sums[i]. 1069 if ei == 1<<logMaxPagesPerSum { 1070 end += 1 << logMaxPagesPerSum 1071 } else { 1072 end = ei 1073 } 1074 } 1075 return packPallocSum(start, most, end) 1076 } 1077