1 /* 2 * Copyright (C) 2022 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef BERBERIS_BASE_MMAP_POOL_H_ 18 #define BERBERIS_BASE_MMAP_POOL_H_ 19 20 #include <atomic> 21 #include <cstddef> 22 23 #include "berberis/base/lock_free_stack.h" 24 #include "berberis/base/logging.h" 25 #include "berberis/base/mmap.h" 26 27 namespace berberis { 28 29 // Pool of memory mappings to be used by berberis runtime. 30 // Thread-safe, signal-safe, reentrant. 31 // Singleton interface (no non-static members). 32 template <size_t kBlockSize, size_t kSizeLimit> 33 class MmapPool { 34 static_assert(kBlockSize % kPageSize == 0); 35 static_assert(kBlockSize <= kSizeLimit); 36 37 public: Alloc()38 static void* Alloc() { 39 Node* node = g_free_list_.Pop(); 40 if (node) { 41 ReleaseFreeListBlock(); 42 return node; 43 } 44 return MmapOrDie(kBlockSize); 45 } 46 Free(void * node)47 static void Free(void* node) { 48 if (AcquireFreeListBlock()) { 49 g_free_list_.Push(static_cast<Node*>(node)); 50 return; 51 } 52 return MunmapOrDie(node, kBlockSize); 53 } 54 55 private: 56 // When a block is freed we reinterpret it as Node, so that 57 // it can be linked to LockFreeStack. 58 struct Node { 59 Node* next; 60 }; 61 ReleaseFreeListBlock()62 static void ReleaseFreeListBlock() { 63 // Memory order release to ensure list pop isn't observed by another 64 // thread after size decrement. 65 g_size_.fetch_sub(kBlockSize, std::memory_order_release); 66 // There must be no more releases than acquires. 67 // On underflow g_size may become close to size_t max. 68 CHECK_LE(g_size_.load(std::memory_order_relaxed), kSizeLimit); 69 } 70 AcquireFreeListBlock()71 static bool AcquireFreeListBlock() { 72 // We need acquire semantics so that list push isn't observed by another 73 // thread before the size reservation. 74 size_t cmp = g_size_.load(std::memory_order_acquire); 75 while (true) { 76 size_t xch = cmp + kBlockSize; 77 // This guarantees that g_size_ is never set to a value above kSizeLimit. 78 if (xch > kSizeLimit) { 79 return false; 80 } 81 // Updates cmp! 82 if (g_size_.compare_exchange_weak(cmp, xch, std::memory_order_acquire)) { 83 return true; 84 } 85 } 86 } 87 88 static LockFreeStack<Node> g_free_list_; 89 // Note, the size is not strictly synchronized with g_free_list_ updates, 90 // but we err on the side of a greater size everywhere to make sure kSizeLimit 91 // isn't overflown. We increase the size *before* pushing a node to list, and 92 // decrease size *after* removing a node. 93 static std::atomic_size_t g_size_; 94 95 MmapPool() = delete; 96 97 friend class MmapPoolTest; 98 }; 99 100 template <size_t kBlockSize, size_t kSizeLimit> 101 std::atomic_size_t MmapPool<kBlockSize, kSizeLimit>::g_size_{0}; 102 103 template <size_t kBlockSize, size_t kSizeLimit> 104 LockFreeStack<typename MmapPool<kBlockSize, kSizeLimit>::Node> 105 MmapPool<kBlockSize, kSizeLimit>::g_free_list_; 106 107 } // namespace berberis 108 109 #endif // BERBERIS_BASE_MMAP_POOL_H_ 110