masm: Don't crash on failed MADV_DONTNEED on Linux

The application could call mlockall(MCL_CURRENT|MCL_FUTURE) to lock all
its memory for performance reasons, causing the madvise call to fail.
There's no need to crash. Instead, manually zero-out the memory when
decommitting.

Fixes: QTBUG-120450
Pick-to: 6.5 6.2 5.15
Change-Id: I6f1a8968853cc5e61561371bd2a435a686eaf0e4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
(cherry picked from commit 524d260)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 1ead022)

Author: napaalm

src/3rdparty/masm/wtf/OSAllocatorPosix.cpp

@@ -114,10 +114,7 @@ void* OSAllocator::reserveUncommitted(size_t bytes, Usage usage, bool writable,
     if (result == MAP_FAILED)
         CRASH();
 
-    while (madvise(result, bytes, MADV_DONTNEED)) {
-        if (errno != EAGAIN)
-            CRASH();
-    }
+    while (madvise(result, bytes, MADV_DONTNEED) == -1 && errno == EAGAIN) { }
 
     if (fd != -1)
         close(fd);
@@ -250,8 +247,10 @@ void OSAllocator::decommit(void* address, size_t bytes)
     mmap(address, bytes, PROT_NONE, MAP_FIXED | MAP_LAZY | MAP_PRIVATE | MAP_ANON, -1, 0);
 #elif OS(LINUX)
     while (madvise(address, bytes, MADV_DONTNEED)) {
-        if (errno != EAGAIN)
-            CRASH();
+        if (errno != EAGAIN) {
+            memset(address, 0, bytes); // We rely on madvise to zero-out the memory
+            break;
+        }
     }
     if (mprotect(address, bytes, PROT_NONE))
         CRASH();