malloc: Do not use MAP_NORESERVE to allocate heap segments

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.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

1 files changed, 0 insertions, 4 deletions

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)