1 // Copyright 2019 The Chromium Authors
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4
5 #ifdef UNSAFE_BUFFERS_BUILD
6 // TODO(crbug.com/40284755): Remove this and spanify to fix the errors.
7 #pragma allow_unsafe_buffers
8 #endif
9
10 #include "base/profiler/stack_buffer.h"
11
12 #include <bit>
13
14 #if BUILDFLAG(IS_CHROMEOS)
15 #include <sys/mman.h>
16
17 #include <ostream>
18
19 #include "base/bits.h"
20 #include "base/check.h"
21 #include "base/check_op.h"
22 #include "base/memory/page_size.h"
23 #endif // #if BUILDFLAG(IS_CHROMEOS)
24
25 namespace base {
26
27 constexpr size_t StackBuffer::kPlatformStackAlignment;
28
29 #if BUILDFLAG(IS_CHROMEOS)
30
MarkUpperBufferContentsAsUnneeded(size_t retained_bytes)31 void StackBuffer::MarkUpperBufferContentsAsUnneeded(size_t retained_bytes) {
32 // Round up to the next multiple of the page size. madvise needs the
33 // starting address to be page aligned. Since buffer_.get() is
34 // already page aligned, we just need to round up the retained bytes.
35 size_t actual_retained_bytes = bits::AlignUp(retained_bytes, GetPageSize());
36
37 // Avoid passing a negative discard_size to madvise(). Doing so would randomly
38 // discard large amounts of memory causing weird crashes.
39 CHECK_LE(actual_retained_bytes, size_);
40
41 uint8_t* start_of_discard =
42 reinterpret_cast<uint8_t*>(buffer_.get()) + actual_retained_bytes;
43 size_t discard_size = size_ - actual_retained_bytes;
44 int result = madvise(start_of_discard, discard_size, MADV_DONTNEED);
45
46 DPCHECK(result == 0) << "madvise failed: ";
47 }
48
49 #endif // #if BUILDFLAG(IS_CHROMEOS)
50
StackBuffer(size_t buffer_size)51 StackBuffer::StackBuffer(size_t buffer_size)
52 #if BUILDFLAG(IS_CHROMEOS)
53 // On ChromeOS, we have 8MB of stack space per thread; however, we normally
54 // only use a small fraction of that. To avoid blowing our memory budget,
55 // we use madvise(MADV_DONTNEED) to let the kernel discard the memory in the
56 // 8MB buffer except when we are actively using it. For madvise() to work,
57 // we need |buffer_| to be aligned to a page boundary.
58 //
59 // We also need the |size_| to be a multiple of the page size so that we
60 // don't pass partial pages to madvise(). This isn't documented but the
61 // program will consistently crash otherwise.
62 : size_(bits::AlignUp(buffer_size, GetPageSize())),
63 buffer_(static_cast<uintptr_t*>(AlignedAlloc(size_, GetPageSize()))) {
64 // Our (very large) buffer may already have data written to it & thus have
65 // backing pages. Tell the kernel we don't need the current contents.
66 MarkUpperBufferContentsAsUnneeded(0);
67 }
68 #else // #if BUILDFLAG(IS_CHROMEOS)
69 : size_(buffer_size),
70 buffer_(static_cast<uintptr_t*>(
71 AlignedAlloc(size_, kPlatformStackAlignment))) {
72 static_assert(std::has_single_bit(kPlatformStackAlignment));
73 }
74 #endif // !#if BUILDFLAG(IS_CHROMEOS)
75
76 StackBuffer::~StackBuffer() = default;
77
78 } // namespace base
79