[PATCH] malloc: Do not use MAP_NORESERVE to allocate heap segments

[Florian Weimer <fweimer@redhat.com>]

Address space for heap segments is reserved in a mmap call with
MAP_ANONYMOUS | MAP_PRIVATE and protection flags PROT_NONE.  This
reservation does not count against the RSS limit of the process or
system.  Backing memory is allocated using mprotect in alloc_new_heap
and grow_heap, and at this point, the allocator expects the kernel
to provide memory (subject to memory overcommit).

The SIGSEGV that might generate due to MAP_NORESERVE (according to
the mmap manual page) does not seem to occur in practice, it's always
SIGKILL from the OOM killer.  Even if there is a way that SIGSEGV
could be generated, it is confusing to applications that this only
happens for secondary heaps, not for large mmap-based allocations,
and not for the main arena.

---
 malloc/arena.c  | 5 +----
 malloc/malloc.c | 4 ----
 2 files changed, 1 insertion(+), 8 deletions(-)

diff --git a/malloc/arena.c b/malloc/arena.c
index defd25c8a6..074ecbc09f 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -559,16 +559,13 @@ new_heap (size_t size, size_t top_pad)
 #if HAVE_TUNABLES
   if (__glibc_unlikely (mp_.hp_pagesize != 0))
     {
-      /* MAP_NORESERVE is not used for huge pages because some kernel may
-	 not reserve the mmap region and a subsequent access may trigger
-	 a SIGBUS if there is no free pages in the pool.  */
       heap_info *h = alloc_new_heap (size, top_pad, mp_.hp_pagesize,
 				     mp_.hp_flags);
       if (h != NULL)
 	return h;
     }
 #endif
-  return alloc_new_heap (size, top_pad, GLRO (dl_pagesize), MAP_NORESERVE);
+  return alloc_new_heap (size, top_pad, GLRO (dl_pagesize), 0);
 }
 
 /* Grow a heap.  size is automatically rounded up to a
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 914052eb69..29fa71b3b2 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1110,10 +1110,6 @@ static mchunkptr mremap_chunk(mchunkptr p, size_t new_size);
 # define MAP_ANONYMOUS MAP_ANON
 #endif
 
-#ifndef MAP_NORESERVE
-# define MAP_NORESERVE 0
-#endif
-
 #define MMAP(addr, size, prot, flags) \
  __mmap((addr), (size), (prot), (flags)|MAP_ANONYMOUS|MAP_PRIVATE, -1, 0)