Check MADV_DONTNEED (for jemalloc), maybe broken under qemu

jemalloc relies on working MADV_DONTNEED (that fact that after
madvise(MADV_DONTNEED) returns success, after subsequent access to those
pages they will be zeroed).
However qemu does not support this, yet [1], and you can get very tricky
assert if you will run clickhouse-server under qemu:

    <jemalloc>: ../contrib/jemalloc/src/extent.c:1195: Failed assertion: "p[i] == 0"

  [1]: https://patchwork.kernel.org/patch/10576637/

But after this patch you will get pretty error:

  $ qemu-x86_64-static programs/clickhouse
  MADV_DONTNEED does not zeroed page. jemalloc will be broken

programs/main.cpp

@@ -2,6 +2,10 @@
 #include <setjmp.h>
 #include <unistd.h>
 
+#ifdef __linux__
+#include <sys/mman.h>
+#endif
+
 #include <new>
 #include <iostream>
 #include <vector>
@@ -296,10 +300,52 @@ void checkRequiredInstructions()
     }
 }
 
+#ifdef __linux__
+/// clickhouse uses jemalloc as a production allocator
+/// and jemalloc relies on working MADV_DONTNEED,
+/// which doesn't work under qemu
+///
+/// but do this only under for linux, since only it return zeroed pages after MADV_DONTNEED
+/// (and jemalloc assumes this too, see contrib/jemalloc-cmake/include_linux_x86_64/jemalloc/internal/jemalloc_internal_defs.h.in)
+void checkRequiredMadviseFlags()
+{
+    size_t size = 1 << 16;
+    void * addr = mmap(nullptr, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+    if (addr == MAP_FAILED)
+    {
+        writeError("Can not mmap pages for MADV_DONTNEED check\n");
+        _Exit(1);
+    }
+    memset(addr, 'A', size);
+
+    if (!madvise(addr, size, MADV_DONTNEED))
+    {
+        /// Suboptimal, but should be simple.
+        for (size_t i = 0; i < size; ++i)
+        {
+            if (reinterpret_cast<unsigned char *>(addr)[i] != 0)
+            {
+                writeError("MADV_DONTNEED does not zeroed page. jemalloc will be broken\n");
+                _Exit(1);
+            }
+        }
+    }
+
+    if (munmap(addr, size))
+    {
+        writeError("Can not munmap pages for MADV_DONTNEED check\n");
+        _Exit(1);
+    }
+}
+#endif
+
 struct Checker
 {
     Checker()
     {
+#ifdef __linux__
+        checkRequiredMadviseFlags();
+#endif
         checkRequiredInstructions();
     }
 } checker;