GCC Code Coverage Report

Directory: cvmfs/
File: cvmfs/malloc_arena.cc
Date: 2025-08-31 02:39:21
Exec Total Coverage
Lines: 121 121 100.0%
Branches: 39 54 72.2%
Line Branch Exec Source
1 /**
2 * This file is part of the CernVM File System.
3 */
4
5 #define __STDC_FORMAT_MACROS
6
7
8 #include "malloc_arena.h"
9
10 #include <cassert>
11 #include <cstddef>
12 #include <cstring>
13 #include <new>
14
15 #include "util/smalloc.h"
16
17 using namespace std; // NOLINT
18
19
20 /**
21 * Walks through the free list starting at rover_ and looks for the first block
22 * larger than block_size. Returns NULL if no such block exists.
23 */
24 279255609 MallocArena::AvailBlockCtl *MallocArena::FindAvailBlock(
25 const int32_t block_size) {
26 279255609 bool wrapped = false;
27 // Generally: p = LINK(q)
28 279255609 AvailBlockCtl *q = rover_;
29 AvailBlockCtl *p;
30 do {
31 102600988174 p = q->GetNextPtr(arena_);
32
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 102600988174 if (p->size >= block_size) {
33 205467232 rover_ = p->GetNextPtr(arena_);
34 205467232 return p;
35 }
36
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 102395520942 if (p == head_avail_) {
37
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 155844502 if (wrapped)
38 73788377 return NULL;
39 82056125 wrapped = true;
40 }
41 102321732565 q = p;
42 } while (true);
43 }
44
45
46 /**
47 * Creates a free block at the place of the reserved block ptr points into.
48 * The free block might need to be merged with adjacent lower and/or upper
49 * blocks. In these cases, the corresponding blocks are removed from the list
50 * of available blocks. Every allocated block has a predecessor and a
51 * successor in the arena. The newly created free block is added to the end of
52 * the list of available blocks.
53 */
54 203027069 void MallocArena::Free(void *ptr) {
55
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 203027069 assert(Contains(ptr));
56
57 203027069 no_reserved_--;
58
59 203027069 ReservedBlockCtl *block_ctl = reinterpret_cast<ReservedBlockCtl *>(
60 reinterpret_cast<char *>(ptr) - sizeof(ReservedBlockCtl));
61 203027069 const char prior_tag = *(reinterpret_cast<char *>(block_ctl) - 1);
62
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 203027069 assert((prior_tag == kTagAvail) || (prior_tag == kTagReserved));
63
64 203027069 int32_t new_size = block_ctl->size();
65
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 203027069 assert(new_size > 0);
66 203027069 AvailBlockCtl *new_avail = reinterpret_cast<AvailBlockCtl *>(block_ctl);
67
68
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 203027069 if (prior_tag == kTagAvail) {
69 // Merge with block before and remove the block from the list
70 74115195 const int32_t prior_size = reinterpret_cast<AvailBlockTag *>(
71 reinterpret_cast<char *>(block_ctl)
72 - sizeof(AvailBlockTag))
73 ->size;
74
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 74115195 assert(prior_size > 0);
75 74115195 new_size += prior_size;
76 74115195 new_avail = reinterpret_cast<AvailBlockCtl *>(
77 74115195 reinterpret_cast<char *>(block_ctl) - prior_size);
78 // new_avail points now to the prior block
79 74115195 UnlinkAvailBlock(new_avail);
80
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 74115195 if (rover_ == new_avail)
81 920571 rover_ = head_avail_;
82 }
83
84 203027069 const int32_t succ_size = *reinterpret_cast<int32_t *>(
85 203027069 reinterpret_cast<char *>(new_avail) + new_size);
86
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 203027069 if (succ_size >= 0) {
87 // Merge with succeeding block and remove the block from the list
88 116002678 AvailBlockCtl *succ_avail = reinterpret_cast<AvailBlockCtl *>(
89 116002678 reinterpret_cast<char *>(new_avail) + new_size);
90 116002678 UnlinkAvailBlock(succ_avail);
91 116002678 new_size += succ_size;
92
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 116002678 if (rover_ == succ_avail)
93 1366508 rover_ = head_avail_;
94 }
95
96 // Set new free block's boundaries
97 203027069 new_avail->size = new_size;
98 203027069 new (AvailBlockTag::GetTagLocation(new_avail)) AvailBlockTag(new_size);
99
100 203027069 EnqueueAvailBlock(new_avail);
101 203027069 }
102
103
104 /**
105 * Inserts an available block at the end of the free list.
106 */
107 203027069 void MallocArena::EnqueueAvailBlock(AvailBlockCtl *block) {
108 203027069 AvailBlockCtl *next = head_avail_;
109 203027069 AvailBlockCtl *prev = head_avail_->GetPrevPtr(arena_);
110 203027069 next->link_prev = block->ConvertToLink(arena_);
111 203027069 prev->link_next = block->ConvertToLink(arena_);
112 203027069 block->link_next = head_avail_->ConvertToLink(arena_);
113 203027069 block->link_prev = prev->ConvertToLink(arena_);
114 203027069 }
115
116
117 /**
118 * The ptr points to the result of Malloc(). The size of the area is stored
119 * a few bytes before ptr.
120 */
121 379084506 uint32_t MallocArena::GetSize(void *ptr) const {
122
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 379084506 assert(Contains(ptr));
123
124 379084506 ReservedBlockCtl *block_ctl = reinterpret_cast<ReservedBlockCtl *>(
125 reinterpret_cast<char *>(ptr) - sizeof(ReservedBlockCtl));
126 379084506 const int32_t size = block_ctl->size();
127
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 379084506 assert(size > 1);
128 379084506 return size - sizeof(ReservedBlockCtl) - 1;
129 }
130
131
132 /**
133 * Walks the list of available blocks starting from rover and allocates the
134 * first available spot that's large enough. Puts the reserved block at the end
135 * of the available one and, if necessary, removes the available one from the
136 * list of free blocks.
137 */
138 279255609 void *MallocArena::Malloc(const uint32_t size) {
139
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 279255609 assert(size > 0);
140
141 // Control word first, block type tag last
142 279255609 int32_t total_size = sizeof(ReservedBlockCtl) + size + 1;
143 279255609 total_size = RoundUp8(total_size);
144
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 279255609 if (total_size < kMinBlockSize)
145 1133169 total_size = kMinBlockSize;
146
147 279255609 AvailBlockCtl *p = FindAvailBlock(total_size);
148
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 279255609 if (p == NULL)
149 73788377 return NULL;
150
151 205467232 no_reserved_++;
152 205467232 return ReserveBlock(p, total_size);
153 }
154
155
156 /**
157 * The arena starts with a pointer to this followed by the AvailBlockCtl of
158 * head_avail_, followed by a reserved tag to prevent it from being merged,
159 * followed by a free block spanning the arena until the end tag. The end tag
160 * is a single negative int, which mimics another reserved block.
161 */
162 3436 MallocArena::MallocArena(unsigned arena_size)
163 3436 : arena_(reinterpret_cast<char *>(sxmmap_align(arena_size)))
164 3436 , head_avail_(reinterpret_cast<AvailBlockCtl *>(arena_ + sizeof(uint64_t)))
165 3436 , rover_(head_avail_)
166 3436 , no_reserved_(0)
167 3436 , arena_size_(arena_size) {
168
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 3436 assert(arena_size_ > 0);
169
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 3436 assert((arena_size_ % (2 * 1024 * 1024)) == 0); // Multiple of 2MB
170
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 3436 assert(arena_size_ <= (512 * 1024 * 1024)); // <= 512MB
171
172 3436 const unsigned char padding = 7;
173 // Size of the initial free block: everything minus arena boundaries
174 3436 const int32_t usable_size = arena_size_
175 3436 - (sizeof(uint64_t) + sizeof(AvailBlockCtl)
176 + padding + 1 + sizeof(int32_t));
177
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 3436 assert((usable_size % 8) == 0);
178
179 // First 8 bytes of arena: this pointer (occupies only 4 bytes on 32bit
180 // architectures, in which case the second 4 bytes are unused.)
181 3436 *reinterpret_cast<MallocArena **>(arena_) = this;
182
183 // The initial large free block
184 3436 AvailBlockCtl *free_block = new (arena_ + sizeof(uint64_t)
185 + sizeof(AvailBlockCtl) + padding + 1)
186 3436 AvailBlockCtl();
187 3436 free_block->size = usable_size;
188 3436 free_block->link_next = free_block->link_prev = head_avail_->ConvertToLink(
189 arena_);
190 3436 new (AvailBlockTag::GetTagLocation(free_block)) AvailBlockTag(usable_size);
191
192 3436 head_avail_->size = 0;
193 3436 head_avail_->link_next = head_avail_->link_prev = free_block->ConvertToLink(
194 arena_);
195
196 // Prevent succeeding blocks from merging
197 3436 *(reinterpret_cast<char *>(free_block) - 1) = kTagReserved;
198 // Final tag: reserved block marker
199 3436 *reinterpret_cast<int32_t *>(arena_ + arena_size_ - sizeof(int32_t)) = -1;
200 3436 }
201
202
203 /**
204 * Initializes the arena with repeated copies of the given pattern. Used for
205 * testing.
206 */
207 48 MallocArena *MallocArena::CreateInitialized(unsigned arena_size,
208 unsigned char pattern) {
209 48 MallocArena *result = new MallocArena(arena_size);
210 // At this point, there is one big free block linked to by head_avail_
211 48 AvailBlockCtl *free_block = result->head_avail_->GetNextPtr(result->arena_);
212
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 48 assert(free_block != result->head_avail_);
213
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 48 assert(free_block->size > 0);
214 // Strip control information at both ends of the block
215 48 const int usable_size = free_block->size
216 48 - (sizeof(AvailBlockCtl) + sizeof(AvailBlockTag));
217
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 48 assert(usable_size > 0);
218 48 memset(free_block + 1, pattern, usable_size);
219 48 return result;
220 }
221
222
223 3431 MallocArena::~MallocArena() { sxunmap(arena_, arena_size_); }
224
225
226 /**
227 * Given the free block "block", cuts out a new reserved block of size
228 * block_size at the end of the free block. Returns a pointer usable by the
229 * application.
230 */
231 205467232 void *MallocArena::ReserveBlock(AvailBlockCtl *block, int32_t block_size) {
232
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 205467232 assert(block->size >= block_size);
233
234 205467232 int32_t remaining_size = block->size - block_size;
235 // Avoid creation of very small blocks
236
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 205467232 if (remaining_size < kMinBlockSize) {
237 12169633 block_size += remaining_size;
238 12169633 remaining_size = 0;
239 }
240
241 // Update the list of available blocks
242
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 205467232 if (remaining_size == 0) {
243 // Remove free block p from the list of available blocks
244 12169633 UnlinkAvailBlock(block);
245 } else {
246 193297599 block->ShrinkTo(remaining_size);
247 }
248
249 // Place the new allocation, which also sets the block type tag at the end
250 205467232 char *new_block = reinterpret_cast<char *>(block) + remaining_size;
251 205467232 new (new_block) ReservedBlockCtl(block_size);
252 205467232 return new_block + sizeof(ReservedBlockCtl);
253 }
254
255
256 /**
257 * Removes the given block from the doubly linked free block list. This happens
258 * when two adjacent free blocks are created in Free() and then merged. Or if
259 * a block gets fully used in Malloc().
260 */
261 202287506 void MallocArena::UnlinkAvailBlock(AvailBlockCtl *block) {
262 202287506 AvailBlockCtl *next = block->GetNextPtr(arena_);
263 202287506 AvailBlockCtl *prev = block->GetPrevPtr(arena_);
264 202287506 prev->link_next = block->link_next;
265 202287506 next->link_prev = block->link_prev;
266 202287506 }
267