From 807a7ff08dc7bf160a8ae0d9f8d416ab3f096558 Mon Sep 17 00:00:00 2001
From: Daan Leijen <daan@microsoft.com>
Date: Mon, 10 Jan 2022 15:39:49 -0800
Subject: [PATCH] improve commit/decommit on Linux

---
 src/os.c | 42 +++++++++++++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 13 deletions(-)

diff --git a/src/os.c b/src/os.c
index 4e0715b1..1939156c 100644
--- a/src/os.c
+++ b/src/os.c
@@ -889,8 +889,7 @@ static bool mi_os_commitx(void* addr, size_t size, bool commit, bool conservativ
 
   #if defined(_WIN32)
   if (commit) {
-    // if the memory was already committed, the call succeeds but it is not zero'd
-    // *is_zero = true;
+    // *is_zero = true;  // note: if the memory was already committed, the call succeeds but the memory is not zero'd
     void* p = VirtualAlloc(start, csize, MEM_COMMIT, PAGE_READWRITE);
     err = (p == start ? 0 : GetLastError());
   }
@@ -900,23 +899,40 @@ static bool mi_os_commitx(void* addr, size_t size, bool commit, bool conservativ
   }
   #elif defined(__wasi__)
   // WebAssembly guests can't control memory protection
-  #elif defined(MAP_FIXED)
-  if (!commit) {
-    // use mmap with MAP_FIXED to discard the existing memory (and reduce commit charge)
-    void* p = mmap(start, csize, PROT_NONE, (MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE), -1, 0);
-    if (p != start) { err = errno; }
-  }
-  else {
-    // for commit, just change the protection
+  #elif 0 && defined(MAP_FIXED) && !defined(__APPLE__)
+  // Linux: disabled for now as mmap fixed seems much more expensive than MADV_DONTNEED (and splits VMA's?)
+  if (commit) {
+    // commit: just change the protection
     err = mprotect(start, csize, (PROT_READ | PROT_WRITE));
     if (err != 0) { err = errno; }
+  } 
+  else {
+    // decommit: use mmap with MAP_FIXED to discard the existing memory (and reduce rss)
+    const int fd = mi_unix_mmap_fd();
+    void* p = mmap(start, csize, PROT_NONE, (MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE), fd, 0);
+    if (p != start) { err = errno; }
+  }
+  #else
+  // Linux, macOSX and others.
+  if (commit) {
+    // commit: ensure we can access the area    
+    err = mprotect(start, csize, (PROT_READ | PROT_WRITE));
+    if (err != 0) { err = errno; }
+  } 
+  else {
+    #if defined(MADV_DONTNEED) && MI_DEBUG == 0 && MI_SECURE == 0
+    // decommit: use MADV_DONTNEED as it decreases rss immediately (unlike MADV_FREE)
+    // (on the other hand, MADV_FREE would be good enough.. it is just not reflected in the stats :-( )
+    err = madvise(start, csize, MADV_DONTNEED);
+    #else
+    // decommit: just disable access (also used in debug and secure mode to trap on illegal access)
+    err = mprotect(start, csize, PROT_NONE);
+    if (err != 0) { err = errno; }
+    #endif
     //#if defined(MADV_FREE_REUSE)
     //  while ((err = mi_madvise(start, csize, MADV_FREE_REUSE)) != 0 && errno == EAGAIN) { errno = 0; }
     //#endif
   }
-  #else
-  err = mprotect(start, csize, (commit ? (PROT_READ | PROT_WRITE) : PROT_NONE));
-  if (err != 0) { err = errno; }
   #endif
   if (err != 0) {
     _mi_warning_message("%s error: start: %p, csize: 0x%zx, err: %i\n", commit ? "commit" : "decommit", start, csize, err);