/* ---------------------------------------------------------------------------- Copyright (c) 2018, Microsoft Research, Daan Leijen This is free software; you can redistribute it and/or modify it under the terms of the MIT license. A copy of the license can be found in the file "LICENSE" at the root of this distribution. -----------------------------------------------------------------------------*/ #ifndef _DEFAULT_SOURCE #define _DEFAULT_SOURCE // ensure mmap flags are defined #endif #include "mimalloc.h" #include "mimalloc-internal.h" #include "mimalloc-atomic.h" #include // strerror #include #if defined(_WIN32) #include #elif defined(__wasi__) // stdlib.h is all we need, and has already been included in mimalloc.h #else #include // mmap #include // sysconf #if defined(__linux__) #include // linux mmap flags #endif #if defined(__APPLE__) #include #endif #endif /* ----------------------------------------------------------- Initialization. On windows initializes support for aligned allocation and large OS pages (if MIMALLOC_LARGE_OS_PAGES is true). ----------------------------------------------------------- */ bool _mi_os_decommit(void* addr, size_t size, mi_stats_t* stats); static void* mi_align_up_ptr(void* p, size_t alignment) { return (void*)_mi_align_up((uintptr_t)p, alignment); } static uintptr_t _mi_align_down(uintptr_t sz, size_t alignment) { return (sz / alignment) * alignment; } static void* mi_align_down_ptr(void* p, size_t alignment) { return (void*)_mi_align_down((uintptr_t)p, alignment); } // page size (initialized properly in `os_init`) static size_t os_page_size = 4096; // minimal allocation granularity static size_t os_alloc_granularity = 4096; // if non-zero, use large page allocation static size_t large_os_page_size = 0; // OS (small) page size size_t _mi_os_page_size() { return os_page_size; } // if large OS pages are supported (2 or 4MiB), then return the size, otherwise return the small page size (4KiB) size_t _mi_os_large_page_size() { return (large_os_page_size != 0 ? large_os_page_size : _mi_os_page_size()); } static bool use_large_os_page(size_t size, size_t alignment) { // if we have access, check the size and alignment requirements if (large_os_page_size == 0 || !mi_option_is_enabled(mi_option_large_os_pages)) return false; return ((size % large_os_page_size) == 0 && (alignment % large_os_page_size) == 0); } // round to a good allocation size static size_t mi_os_good_alloc_size(size_t size, size_t alignment) { UNUSED(alignment); if (size >= (SIZE_MAX - os_alloc_granularity)) return size; // possible overflow? return _mi_align_up(size, os_alloc_granularity); } #if defined(_WIN32) // We use VirtualAlloc2 for aligned allocation, but it is only supported on Windows 10 and Windows Server 2016. // So, we need to look it up dynamically to run on older systems. (use __stdcall for 32-bit compatibility) // (hide MEM_EXTENDED_PARAMETER to compile with older SDK's) typedef PVOID(__stdcall *PVirtualAlloc2)(HANDLE, PVOID, SIZE_T, ULONG, ULONG, /* MEM_EXTENDED_PARAMETER* */ void*, ULONG); static PVirtualAlloc2 pVirtualAlloc2 = NULL; void _mi_os_init(void) { // get the page size SYSTEM_INFO si; GetSystemInfo(&si); if (si.dwPageSize > 0) os_page_size = si.dwPageSize; if (si.dwAllocationGranularity > 0) os_alloc_granularity = si.dwAllocationGranularity; // get the VirtualAlloc2 function HINSTANCE hDll; hDll = LoadLibrary(TEXT("kernelbase.dll")); if (hDll != NULL) { // use VirtualAlloc2FromApp if possible as it is available to Windows store apps pVirtualAlloc2 = (PVirtualAlloc2)GetProcAddress(hDll, "VirtualAlloc2FromApp"); if (pVirtualAlloc2==NULL) pVirtualAlloc2 = (PVirtualAlloc2)GetProcAddress(hDll, "VirtualAlloc2"); FreeLibrary(hDll); } // Try to see if large OS pages are supported unsigned long err = 0; bool ok = mi_option_is_enabled(mi_option_large_os_pages); if (ok) { // To use large pages on Windows, we first need access permission // Set "Lock pages in memory" permission in the group policy editor // HANDLE token = NULL; ok = OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &token); if (ok) { TOKEN_PRIVILEGES tp; ok = LookupPrivilegeValue(NULL, TEXT("SeLockMemoryPrivilege"), &tp.Privileges[0].Luid); if (ok) { tp.PrivilegeCount = 1; tp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED; ok = AdjustTokenPrivileges(token, FALSE, &tp, 0, (PTOKEN_PRIVILEGES)NULL, 0); if (ok) { err = GetLastError(); ok = (err == ERROR_SUCCESS); if (ok) { large_os_page_size = GetLargePageMinimum(); } } } CloseHandle(token); } if (!ok) { if (err == 0) err = GetLastError(); _mi_warning_message("cannot enable large OS page support, error %lu\n", err); } } } #elif defined(__wasi__) void _mi_os_init() { os_page_size = 0x10000; // WebAssembly has a fixed page size: 64KB os_alloc_granularity = 16; } #else void _mi_os_init() { // get the page size long result = sysconf(_SC_PAGESIZE); if (result > 0) { os_page_size = (size_t)result; os_alloc_granularity = os_page_size; } if (mi_option_is_enabled(mi_option_large_os_pages)) { large_os_page_size = (1UL << 21); // 2MiB } } #endif /* ----------------------------------------------------------- Raw allocation on Windows (VirtualAlloc) and Unix's (mmap). ----------------------------------------------------------- */ static bool mi_os_mem_free(void* addr, size_t size, mi_stats_t* stats) { if (addr == NULL || size == 0) return true; bool err = false; #if defined(_WIN32) err = (VirtualFree(addr, 0, MEM_RELEASE) == 0); #elif defined(__wasi__) err = 0; // WebAssembly's heap cannot be shrunk #else err = (munmap(addr, size) == -1); #endif _mi_stat_decrease(&stats->committed, size); // TODO: what if never committed? _mi_stat_decrease(&stats->reserved, size); if (err) { #pragma warning(suppress:4996) _mi_warning_message("munmap failed: %s, addr 0x%8li, size %lu\n", strerror(errno), (size_t)addr, size); return false; } else { return true; } } #ifdef _WIN32 static void* mi_win_virtual_allocx(void* addr, size_t size, size_t try_alignment, DWORD flags) { #if (MI_INTPTR_SIZE >= 8) // on 64-bit systems, use the virtual address area after 4TiB for 4MiB aligned allocations static volatile intptr_t aligned_base = ((intptr_t)4 << 40); // starting at 4TiB if (addr == NULL && try_alignment > 0 && try_alignment <= MI_SEGMENT_SIZE && (size%MI_SEGMENT_SIZE) == 0) { intptr_t hint = mi_atomic_add(&aligned_base, size) - size; if (hint%try_alignment == 0) { return VirtualAlloc((void*)hint, size, flags, PAGE_READWRITE); } } #endif #if defined(MEM_EXTENDED_PARAMETER_TYPE_BITS) if (try_alignment > 0 && (try_alignment % _mi_os_page_size()) == 0 && pVirtualAlloc2 != NULL) { // on modern Windows try use VirtualAlloc2 for aligned allocation MEM_ADDRESS_REQUIREMENTS reqs = { 0 }; reqs.Alignment = try_alignment; MEM_EXTENDED_PARAMETER param = { 0 }; param.Type = MemExtendedParameterAddressRequirements; param.Pointer = &reqs; return (*pVirtualAlloc2)(addr, NULL, size, flags, PAGE_READWRITE, ¶m, 1); } #endif return VirtualAlloc(addr, size, flags, PAGE_READWRITE); } static void* mi_win_virtual_alloc(void* addr, size_t size, size_t try_alignment, DWORD flags) { static volatile uintptr_t large_page_try_ok = 0; void* p = NULL; if (use_large_os_page(size, try_alignment)) { uintptr_t try_ok = mi_atomic_read(&large_page_try_ok); if (try_ok > 0) { // if a large page allocation fails, it seems the calls to VirtualAlloc get very expensive. // therefore, once a large page allocation failed, we don't try again for `large_page_try_ok` times. mi_atomic_compare_exchange(&large_page_try_ok, try_ok - 1, try_ok); } else { // large OS pages must always reserve and commit. p = mi_win_virtual_allocx(addr, size, try_alignment, MEM_LARGE_PAGES | MEM_COMMIT | MEM_RESERVE | flags); // fall back to non-large page allocation on error (`p == NULL`). if (p == NULL) { mi_atomic_write(&large_page_try_ok,10); // on error, don't try again for the next N allocations } } } if (p == NULL) { p = mi_win_virtual_allocx(addr, size, try_alignment, flags); } return p; } #elif defined(__wasi__) static void* mi_wasm_heap_grow(size_t size, size_t try_alignment) { uintptr_t base = __builtin_wasm_memory_size(0) * _mi_os_page_size(); uintptr_t aligned_base = _mi_align_up(base, (uintptr_t) try_alignment); size_t alloc_size = _mi_align_up( aligned_base - base + size, _mi_os_page_size()); mi_assert(alloc_size >= size && (alloc_size % _mi_os_page_size()) == 0); if (alloc_size < size) return NULL; if (__builtin_wasm_memory_grow(0, alloc_size / _mi_os_page_size()) == SIZE_MAX) { errno = ENOMEM; return NULL; } return (void*)aligned_base; } #else static void* mi_unix_mmapx(size_t size, size_t try_alignment, int protect_flags, int flags, int fd) { void* p = NULL; #if (MI_INTPTR_SIZE >= 8) && !defined(MAP_ALIGNED) // on 64-bit systems, use the virtual address area after 4TiB for 4MiB aligned allocations static volatile intptr_t aligned_base = ((intptr_t)1 << 42); // starting at 4TiB if (try_alignment <= MI_SEGMENT_SIZE && (size%MI_SEGMENT_SIZE)==0) { intptr_t hint = mi_atomic_add(&aligned_base,size) - size; if (hint%try_alignment == 0) { p = mmap((void*)hint,size,protect_flags,flags,fd,0); if (p==MAP_FAILED) p = NULL; // fall back to regular mmap } } #endif if (p==NULL) { p = mmap(NULL,size,protect_flags,flags,fd,0); } return p; } static void* mi_unix_mmap(size_t size, size_t try_alignment, int protect_flags) { void* p = NULL; #if !defined(MAP_ANONYMOUS) #define MAP_ANONYMOUS MAP_ANON #endif int flags = MAP_PRIVATE | MAP_ANONYMOUS; int fd = -1; #if defined(MAP_ALIGNED) // BSD if (try_alignment > 0) { size_t n = _mi_bsr(try_alignment); if (((size_t)1 << n) == try_alignment && n >= 12 && n <= 30) { // alignment is a power of 2 and 4096 <= alignment <= 1GiB flags |= MAP_ALIGNED(n); } } #endif #if defined(PROT_MAX) protect_flags |= PROT_MAX(PROT_READ | PROT_WRITE); // BSD #endif #if defined(VM_MAKE_TAG) // macOS: tracking anonymous page with a specific ID. (All up to 98 are taken officially but LLVM sanitizers had taken 99) fd = VM_MAKE_TAG(100); #endif if (use_large_os_page(size, try_alignment)) { static volatile uintptr_t large_page_try_ok = 0; uintptr_t try_ok = mi_atomic_read(&large_page_try_ok); if (try_ok > 0) { // If the OS is not configured for large OS pages, or the user does not have // enough permission, the `mmap` will always fail (but it might also fail for other reasons). // Therefore, once a large page allocation failed, we don't try again for `large_page_try_ok` times // to avoid too many failing calls to mmap. mi_atomic_compare_exchange(&large_page_try_ok, try_ok - 1, try_ok); } else { int lflags = flags; int lfd = fd; #ifdef MAP_ALIGNED_SUPER lflags |= MAP_ALIGNED_SUPER; #endif #ifdef MAP_HUGETLB lflags |= MAP_HUGETLB; #endif #ifdef MAP_HUGE_2MB lflags |= MAP_HUGE_2MB; #endif #ifdef VM_FLAGS_SUPERPAGE_SIZE_2MB lfd |= VM_FLAGS_SUPERPAGE_SIZE_2MB; #endif if (lflags != flags) { // try large OS page allocation p = mi_unix_mmapx(size, try_alignment, protect_flags, lflags, lfd); if (p == MAP_FAILED) { mi_atomic_write(&large_page_try_ok, 10); // on error, don't try again for the next N allocations p = NULL; // and fall back to regular mmap } } } } if (p == NULL) { p = mi_unix_mmapx(size, try_alignment, protect_flags, flags, fd); if (p == MAP_FAILED) { p = NULL; } #if defined(MADV_HUGEPAGE) // Many Linux systems don't allow MAP_HUGETLB but they support instead // transparent huge pages (TPH). It is not required to call `madvise` with MADV_HUGE // though since properly aligned allocations will already use large pages if available // in that case -- in particular for our large regions (in `memory.c`). // However, some systems only allow TPH if called with explicit `madvise`, so // when large OS pages are enabled for mimalloc, we call `madvice` anyways. else if (use_large_os_page(size, try_alignment)) { madvise(p, size, MADV_HUGEPAGE); } #endif } return p; } #endif // Primitive allocation from the OS. // Note: the `try_alignment` is just a hint and the returned pointer is not guaranteed to be aligned. static void* mi_os_mem_alloc(size_t size, size_t try_alignment, bool commit, mi_stats_t* stats) { mi_assert_internal(size > 0 && (size % _mi_os_page_size()) == 0); if (size == 0) return NULL; void* p = NULL; #if defined(_WIN32) int flags = MEM_RESERVE; if (commit) flags |= MEM_COMMIT; p = mi_win_virtual_alloc(NULL, size, try_alignment, flags); #elif defined(__wasi__) p = mi_wasm_heap_grow(size, try_alignment); #else int protect_flags = (commit ? (PROT_WRITE | PROT_READ) : PROT_NONE); p = mi_unix_mmap(size, try_alignment, protect_flags); #endif _mi_stat_increase(&stats->mmap_calls, 1); if (p != NULL) { _mi_stat_increase(&stats->reserved, size); if (commit) _mi_stat_increase(&stats->committed, size); } return p; } // Primitive aligned allocation from the OS. // This function guarantees the allocated memory is aligned. static void* mi_os_mem_alloc_aligned(size_t size, size_t alignment, bool commit, mi_stats_t* stats) { mi_assert_internal(alignment >= _mi_os_page_size() && ((alignment & (alignment - 1)) == 0)); mi_assert_internal(size > 0 && (size % _mi_os_page_size()) == 0); if (!(alignment >= _mi_os_page_size() && ((alignment & (alignment - 1)) == 0))) return NULL; size = _mi_align_up(size, _mi_os_page_size()); // try first with a hint (this will be aligned directly on Win 10+ or BSD) void* p = mi_os_mem_alloc(size, alignment, commit, stats); if (p == NULL) return NULL; // if not aligned, free it, overallocate, and unmap around it if (((uintptr_t)p % alignment != 0)) { mi_os_mem_free(p, size, stats); if (size >= (SIZE_MAX - alignment)) return NULL; // overflow size_t over_size = size + alignment; #if _WIN32 // over-allocate and than re-allocate exactly at an aligned address in there. // this may fail due to threads allocating at the same time so we // retry this at most 3 times before giving up. // (we can not decommit around the overallocation on Windows, because we can only // free the original pointer, not one pointing inside the area) int flags = MEM_RESERVE; if (commit) flags |= MEM_COMMIT; for (int tries = 0; tries < 3; tries++) { // over-allocate to determine a virtual memory range p = mi_os_mem_alloc(over_size, alignment, commit, stats); if (p == NULL) return NULL; // error if (((uintptr_t)p % alignment) == 0) { // if p happens to be aligned, just decommit the left-over area _mi_os_decommit((uint8_t*)p + size, over_size - size, stats); break; } else { // otherwise free and allocate at an aligned address in there mi_os_mem_free(p, over_size, stats); void* aligned_p = mi_align_up_ptr(p, alignment); p = mi_win_virtual_alloc(aligned_p, size, alignment, flags); if (p == aligned_p) break; // success! if (p != NULL) { // should not happen? mi_os_mem_free(p, size, stats); p = NULL; } } } #else // overallocate... p = mi_os_mem_alloc(over_size, alignment, commit, stats); if (p == NULL) return NULL; // and selectively unmap parts around the over-allocated area. void* aligned_p = mi_align_up_ptr(p, alignment); size_t pre_size = (uint8_t*)aligned_p - (uint8_t*)p; size_t mid_size = _mi_align_up(size, _mi_os_page_size()); size_t post_size = over_size - pre_size - mid_size; mi_assert_internal(pre_size < over_size && post_size < over_size && mid_size >= size); if (pre_size > 0) mi_os_mem_free(p, pre_size, stats); if (post_size > 0) mi_os_mem_free((uint8_t*)aligned_p + mid_size, post_size, stats); // we can return the aligned pointer on `mmap` systems p = aligned_p; #endif } mi_assert_internal(p == NULL || (p != NULL && ((uintptr_t)p % alignment) == 0)); return p; } /* ----------------------------------------------------------- OS API: alloc, free, alloc_aligned ----------------------------------------------------------- */ void* _mi_os_alloc(size_t size, mi_stats_t* stats) { if (size == 0) return NULL; size = mi_os_good_alloc_size(size, 0); return mi_os_mem_alloc(size, 0, true, stats); } void _mi_os_free(void* p, size_t size, mi_stats_t* stats) { if (size == 0 || p == NULL) return; size = mi_os_good_alloc_size(size, 0); mi_os_mem_free(p, size, stats); } void* _mi_os_alloc_aligned(size_t size, size_t alignment, bool commit, mi_os_tld_t* tld) { if (size == 0) return NULL; size = mi_os_good_alloc_size(size, alignment); alignment = _mi_align_up(alignment, _mi_os_page_size()); return mi_os_mem_alloc_aligned(size, alignment, commit, tld->stats); } /* ----------------------------------------------------------- OS memory API: reset, commit, decommit, protect, unprotect. ----------------------------------------------------------- */ // OS page align within a given area, either conservative (pages inside the area only), // or not (straddling pages outside the area is possible) static void* mi_os_page_align_areax(bool conservative, void* addr, size_t size, size_t* newsize) { mi_assert(addr != NULL && size > 0); if (newsize != NULL) *newsize = 0; if (size == 0 || addr == NULL) return NULL; // page align conservatively within the range void* start = (conservative ? mi_align_up_ptr(addr, _mi_os_page_size()) : mi_align_down_ptr(addr, _mi_os_page_size())); void* end = (conservative ? mi_align_down_ptr((uint8_t*)addr + size, _mi_os_page_size()) : mi_align_up_ptr((uint8_t*)addr + size, _mi_os_page_size())); ptrdiff_t diff = (uint8_t*)end - (uint8_t*)start; if (diff <= 0) return NULL; mi_assert_internal((conservative && (size_t)diff <= size) || (!conservative && (size_t)diff >= size)); if (newsize != NULL) *newsize = (size_t)diff; return start; } static void* mi_os_page_align_area_conservative(void* addr, size_t size, size_t* newsize) { return mi_os_page_align_areax(true, addr, size, newsize); } // Commit/Decommit memory. // Usuelly commit is aligned liberal, while decommit is aligned conservative. // (but not for the reset version where we want commit to be conservative as well) static bool mi_os_commitx(void* addr, size_t size, bool commit, bool conservative, mi_stats_t* stats) { // page align in the range, commit liberally, decommit conservative size_t csize; void* start = mi_os_page_align_areax(conservative, addr, size, &csize); if (csize == 0) return true; int err = 0; if (commit) { _mi_stat_increase(&stats->committed, csize); _mi_stat_increase(&stats->commit_calls, 1); } else { _mi_stat_decrease(&stats->committed, csize); } #if defined(_WIN32) if (commit) { void* p = VirtualAlloc(start, csize, MEM_COMMIT, PAGE_READWRITE); err = (p == start ? 0 : GetLastError()); } else { BOOL ok = VirtualFree(start, csize, MEM_DECOMMIT); err = (ok ? 0 : GetLastError()); } #elif defined(__wasi__) // WebAssembly guests can't control memory protection #else err = mprotect(start, csize, (commit ? (PROT_READ | PROT_WRITE) : PROT_NONE)); #endif if (err != 0) { _mi_warning_message("commit/decommit error: start: 0x%p, csize: 0x%x, err: %i\n", start, csize, err); } mi_assert_internal(err == 0); return (err == 0); } bool _mi_os_commit(void* addr, size_t size, mi_stats_t* stats) { return mi_os_commitx(addr, size, true, false /* conservative? */, stats); } bool _mi_os_decommit(void* addr, size_t size, mi_stats_t* stats) { return mi_os_commitx(addr, size, false, true /* conservative? */, stats); } bool _mi_os_commit_unreset(void* addr, size_t size, mi_stats_t* stats) { return mi_os_commitx(addr, size, true, true /* conservative? */, stats); } // Signal to the OS that the address range is no longer in use // but may be used later again. This will release physical memory // pages and reduce swapping while keeping the memory committed. // We page align to a conservative area inside the range to reset. static bool mi_os_resetx(void* addr, size_t size, bool reset, mi_stats_t* stats) { // page align conservatively within the range size_t csize; void* start = mi_os_page_align_area_conservative(addr, size, &csize); if (csize == 0) return true; if (reset) _mi_stat_increase(&stats->reset, csize); else _mi_stat_decrease(&stats->reset, csize); if (!reset) return true; // nothing to do on unreset! #if (MI_DEBUG>1) if (!mi_option_is_enabled(mi_option_secure)) { memset(start, 0, csize); // pretend it is eagerly reset } #endif #if defined(_WIN32) // Testing shows that for us (on `malloc-large`) MEM_RESET is 2x faster than DiscardVirtualMemory void* p = VirtualAlloc(start, csize, MEM_RESET, PAGE_READWRITE); mi_assert_internal(p == start); if (p != start) return false; #else #if defined(MADV_FREE) static int advice = MADV_FREE; int err = madvise(start, csize, advice); if (err != 0 && errno == EINVAL && advice == MADV_FREE) { // if MADV_FREE is not supported, fall back to MADV_DONTNEED from now on advice = MADV_DONTNEED; err = madvise(start, csize, advice); } #elif defined(__wasi__) int err = 0; #else int err = madvise(start, csize, MADV_DONTNEED); #endif if (err != 0) { _mi_warning_message("madvise reset error: start: 0x%p, csize: 0x%x, errno: %i\n", start, csize, errno); } //mi_assert(err == 0); if (err != 0) return false; #endif return true; } // Signal to the OS that the address range is no longer in use // but may be used later again. This will release physical memory // pages and reduce swapping while keeping the memory committed. // We page align to a conservative area inside the range to reset. bool _mi_os_reset(void* addr, size_t size, mi_stats_t* stats) { if (mi_option_is_enabled(mi_option_reset_decommits)) { return _mi_os_decommit(addr,size,stats); } else { return mi_os_resetx(addr, size, true, stats); } } bool _mi_os_unreset(void* addr, size_t size, mi_stats_t* stats) { if (mi_option_is_enabled(mi_option_reset_decommits)) { return _mi_os_commit_unreset(addr, size, stats); // re-commit it (conservatively!) } else { return mi_os_resetx(addr, size, false, stats); } } // Protect a region in memory to be not accessible. static bool mi_os_protectx(void* addr, size_t size, bool protect) { // page align conservatively within the range size_t csize = 0; void* start = mi_os_page_align_area_conservative(addr, size, &csize); if (csize == 0) return false; int err = 0; #ifdef _WIN32 DWORD oldprotect = 0; BOOL ok = VirtualProtect(start, csize, protect ? PAGE_NOACCESS : PAGE_READWRITE, &oldprotect); err = (ok ? 0 : GetLastError()); #elif defined(__wasi__) err = 0; #else err = mprotect(start, csize, protect ? PROT_NONE : (PROT_READ | PROT_WRITE)); #endif if (err != 0) { _mi_warning_message("mprotect error: start: 0x%p, csize: 0x%x, err: %i\n", start, csize, err); } return (err == 0); } bool _mi_os_protect(void* addr, size_t size) { return mi_os_protectx(addr, size, true); } bool _mi_os_unprotect(void* addr, size_t size) { return mi_os_protectx(addr, size, false); } bool _mi_os_shrink(void* p, size_t oldsize, size_t newsize, mi_stats_t* stats) { // page align conservatively within the range mi_assert_internal(oldsize > newsize && p != NULL); if (oldsize < newsize || p == NULL) return false; if (oldsize == newsize) return true; // oldsize and newsize should be page aligned or we cannot shrink precisely void* addr = (uint8_t*)p + newsize; size_t size = 0; void* start = mi_os_page_align_area_conservative(addr, oldsize - newsize, &size); if (size == 0 || start != addr) return false; #ifdef _WIN32 // we cannot shrink on windows, but we can decommit return _mi_os_decommit(start, size, stats); #else return mi_os_mem_free(start, size, stats); #endif }