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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package runtime
import (
"runtime/internal/sys"
"unsafe"
)
const pageCachePages = 8 * unsafe.Sizeof(pageCache{}.cache)
// pageCache represents a per-p cache of pages the allocator can
// allocate from without a lock. More specifically, it represents
// a pageCachePages*pageSize chunk of memory with 0 or more free
// pages in it.
type pageCache struct {
base uintptr // base address of the chunk
cache uint64 // 64-bit bitmap representing free pages (1 means free)
scav uint64 // 64-bit bitmap representing scavenged pages (1 means scavenged)
}
// empty reports whether the page cache has no free pages.
func (c *pageCache) empty() bool {
return c.cache == 0
}
// alloc allocates npages from the page cache and is the main entry
// point for allocation.
//
// Returns a base address and the amount of scavenged memory in the
// allocated region in bytes.
//
// Returns a base address of zero on failure, in which case the
// amount of scavenged memory should be ignored.
func (c *pageCache) alloc(npages uintptr) (uintptr, uintptr) {
if c.cache == 0 {
return 0, 0
}
if npages == 1 {
i := uintptr(sys.TrailingZeros64(c.cache))
scav := (c.scav >> i) & 1
c.cache &^= 1 << i // set bit to mark in-use
c.scav &^= 1 << i // clear bit to mark unscavenged
return c.base + i*pageSize, uintptr(scav) * pageSize
}
return c.allocN(npages)
}
// allocN is a helper which attempts to allocate npages worth of pages
// from the cache. It represents the general case for allocating from
// the page cache.
//
// Returns a base address and the amount of scavenged memory in the
// allocated region in bytes.
func (c *pageCache) allocN(npages uintptr) (uintptr, uintptr) {
i := findBitRange64(c.cache, uint(npages))
if i >= 64 {
return 0, 0
}
mask := ((uint64(1) << npages) - 1) << i
scav := sys.OnesCount64(c.scav & mask)
c.cache &^= mask // mark in-use bits
c.scav &^= mask // clear scavenged bits
return c.base + uintptr(i*pageSize), uintptr(scav) * pageSize
}
// flush empties out unallocated free pages in the given cache
// into s. Then, it clears the cache, such that empty returns
// true.
//
// p.mheapLock must be held.
//
// Must run on the system stack because p.mheapLock must be held.
//
//go:systemstack
func (c *pageCache) flush(p *pageAlloc) {
assertLockHeld(p.mheapLock)
if c.empty() {
return
}
ci := chunkIndex(c.base)
pi := chunkPageIndex(c.base)
// This method is called very infrequently, so just do the
// slower, safer thing by iterating over each bit individually.
for i := uint(0); i < 64; i++ {
if c.cache&(1<<i) != 0 {
p.chunkOf(ci).free1(pi + i)
}
if c.scav&(1<<i) != 0 {
p.chunkOf(ci).scavenged.setRange(pi+i, 1)
}
}
// Since this is a lot like a free, we need to make sure
// we update the searchAddr just like free does.
if b := (offAddr{c.base}); b.lessThan(p.searchAddr) {
p.searchAddr = b
}
p.update(c.base, pageCachePages, false, false)
*c = pageCache{}
}
// allocToCache acquires a pageCachePages-aligned chunk of free pages which
// may not be contiguous, and returns a pageCache structure which owns the
// chunk.
//
// p.mheapLock must be held.
//
// Must run on the system stack because p.mheapLock must be held.
//
//go:systemstack
func (p *pageAlloc) allocToCache() pageCache {
assertLockHeld(p.mheapLock)
// If the searchAddr refers to a region which has a higher address than
// any known chunk, then we know we're out of memory.
if chunkIndex(p.searchAddr.addr()) >= p.end {
return pageCache{}
}
c := pageCache{}
ci := chunkIndex(p.searchAddr.addr()) // chunk index
var chunk *pallocData
if p.summary[len(p.summary)-1][ci] != 0 {
// Fast path: there's free pages at or near the searchAddr address.
chunk = p.chunkOf(ci)
j, _ := chunk.find(1, chunkPageIndex(p.searchAddr.addr()))
if j == ^uint(0) {
throw("bad summary data")
}
c = pageCache{
base: chunkBase(ci) + alignDown(uintptr(j), 64)*pageSize,
cache: ^chunk.pages64(j),
scav: chunk.scavenged.block64(j),
}
} else {
// Slow path: the searchAddr address had nothing there, so go find
// the first free page the slow way.
addr, _ := p.find(1)
if addr == 0 {
// We failed to find adequate free space, so mark the searchAddr as OoM
// and return an empty pageCache.
p.searchAddr = maxSearchAddr()
return pageCache{}
}
ci := chunkIndex(addr)
chunk = p.chunkOf(ci)
c = pageCache{
base: alignDown(addr, 64*pageSize),
cache: ^chunk.pages64(chunkPageIndex(addr)),
scav: chunk.scavenged.block64(chunkPageIndex(addr)),
}
}
// Set the page bits as allocated and clear the scavenged bits, but
// be careful to only set and clear the relevant bits.
cpi := chunkPageIndex(c.base)
chunk.allocPages64(cpi, c.cache)
chunk.scavenged.clearBlock64(cpi, c.cache&c.scav /* free and scavenged */)
// Update as an allocation, but note that it's not contiguous.
p.update(c.base, pageCachePages, false, true)
// Set the search address to the last page represented by the cache.
// Since all of the pages in this block are going to the cache, and we
// searched for the first free page, we can confidently start at the
// next page.
//
// However, p.searchAddr is not allowed to point into unmapped heap memory
// unless it is maxSearchAddr, so make it the last page as opposed to
// the page after.
p.searchAddr = offAddr{c.base + pageSize*(pageCachePages-1)}
return c
}
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