Support older linux systems (#545)

* Fix pal_linux.h for older linux systems

Where MADV_FREE is not defined - replaced with MADV_DONTNEED
Where GRND_NONBLOCK is not defined in <sys/random.h> but in <linux/random.h>

* Check for linux/random.h in CMake

as __has_include seems to not be reliable

* Use CMake module CheckIncludeFilesCXX

as C language isn't enabled by default everywhere

* Move madvise flag ifdefs into constexpr for cleaner code

CMakeLists.txt

@@ -8,6 +8,7 @@ endif()
 
 include(CheckCXXCompilerFlag)
 include(CheckCXXSourceCompiles)
+include(CheckIncludeFileCXX)
 include(CMakeDependentOption)
 
 option(SNMALLOC_HEADER_ONLY_LIBRARY "Use snmalloc has a header-only library" OFF)
@@ -105,6 +106,14 @@ int main() {
   return res;
 }
 " SNMALLOC_PLATFORM_HAS_GETENTROPY)
+
+# check if linux/random.h is available
+# older libcs might not have sys/random.h
+# but some might provide the necessary flags via linux/random.h
+# the __has_include macro isn't working properly on all platforms for that header
+# this is why we check its existence here
+CHECK_INCLUDE_FILE_CXX(linux/random.h SNMALLOC_HAS_LINUX_RANDOM_H)
+
 # Provide as function so other projects can reuse
 # FIXME: This modifies some variables that may or may not be the ones that
 # provide flags and so is broken by design.  It should be removed once Verona
@@ -203,6 +212,7 @@ add_as_define(SNMALLOC_QEMU_WORKAROUND)
 add_as_define(SNMALLOC_TRACING)
 add_as_define(SNMALLOC_CI_BUILD)
 add_as_define(SNMALLOC_PLATFORM_HAS_GETENTROPY)
+add_as_define(SNMALLOC_HAS_LINUX_RANDOM_H)
 if (SNMALLOC_NO_REALLOCARRAY)
   add_as_define(SNMALLOC_NO_REALLOCARRAY)
 endif()

src/snmalloc/pal/pal_linux.h

@@ -8,6 +8,11 @@
 #  include <sys/mman.h>
 #  include <sys/prctl.h>
 #  include <syscall.h>
+// __has_include does not reliably determine if we actually have linux/random.h
+// available
+#  if defined(SNMALLOC_HAS_LINUX_RANDOM_H)
+#    include <linux/random.h>
+#  endif
 
 extern "C" int puts(const char* str);
 
@@ -35,6 +40,19 @@ namespace snmalloc
      */
     static constexpr int default_mmap_flags = MAP_NORESERVE;
 
+    /**
+     * MADV_FREE is only available since Linux 4.5.
+     *
+     * Fallback to MADV_DONTNEED on older kernels
+     */
+    static constexpr int madvise_free_flags =
+#  ifdef SNMALLOC_HAS_LINUX_RANDOM_H
+      MADV_FREE
+#  else
+      MADV_DONTNEED
+#  endif
+      ;
+
     static void* reserve(size_t size) noexcept
     {
       void* p = PALPOSIX<PALLinux>::reserve(size);
@@ -108,7 +126,7 @@ namespace snmalloc
         memset(p, 0x5a, size);
 
       madvise(p, size, MADV_DONTDUMP);
-      madvise(p, size, MADV_FREE);
+      madvise(p, size, madvise_free_flags);
 
       if constexpr (PalEnforceAccess)
       {