Fix chunk_alloc_mmap() bugs.

Simplify chunk_alloc_mmap() to no longer attempt map extension.  The
extra complexity isn't warranted, because although in the success case
it saves one system call as compared to immediately falling back to
chunk_alloc_mmap_slow(), it also makes the failure case even more
expensive.  This simplification removes two bugs:

- For Windows platforms, pages_unmap() wasn't being called for unaligned
  mappings prior to falling back to chunk_alloc_mmap_slow().  This
  caused permanent virtual memory leaks.
- For non-Windows platforms, alignment greater than chunksize caused
  pages_map() to be called with size 0 when attempting map extension.
  This always resulted in an mmap() error, and subsequent fallback to
  chunk_alloc_mmap_slow().

src/chunk.c

@@ -134,6 +134,7 @@ chunk_alloc(size_t size, size_t alignment, bool base, bool *zero)
 
 	assert(size != 0);
 	assert((size & chunksize_mask) == 0);
+	assert(alignment != 0);
 	assert((alignment & chunksize_mask) == 0);
 
 	ret = chunk_recycle(size, alignment, base, zero);

src/chunk_mmap.c

@@ -16,6 +16,8 @@ pages_map(void *addr, size_t size)
 {
 	void *ret;
 
+	assert(size != 0);
+
 #ifdef _WIN32
 	/*
 	 * If VirtualAlloc can't allocate at the given address when one is
@@ -164,51 +166,24 @@ chunk_alloc_mmap(size_t size, size_t alignment, bool *zero)
 	 * NetBSD has), but in the absence of such a feature, we have to work
 	 * hard to efficiently create aligned mappings.  The reliable, but
 	 * slow method is to create a mapping that is over-sized, then trim the
-	 * excess.  However, that always results in at least one call to
+	 * excess.  However, that always results in one or two calls to
 	 * pages_unmap().
 	 *
-	 * A more optimistic approach is to try mapping precisely the right
-	 * amount, then try to append another mapping if alignment is off.  In
-	 * practice, this works out well as long as the application is not
-	 * interleaving mappings via direct mmap() calls.  If we do run into a
-	 * situation where there is an interleaved mapping and we are unable to
-	 * extend an unaligned mapping, our best option is to switch to the
-	 * slow method until mmap() returns another aligned mapping.  This will
-	 * tend to leave a gap in the memory map that is too small to cause
-	 * later problems for the optimistic method.
-	 *
-	 * Another possible confounding factor is address space layout
-	 * randomization (ASLR), which causes mmap(2) to disregard the
-	 * requested address.  As such, repeatedly trying to extend unaligned
-	 * mappings could result in an infinite loop, so if extension fails,
-	 * immediately fall back to the reliable method of over-allocation
-	 * followed by trimming.
+	 * Optimistically try mapping precisely the right amount before falling
+	 * back to the slow method, with the expectation that the optimistic
+	 * approach works most of the time.
 	 */
 
+	assert(alignment != 0);
+	assert((alignment & chunksize_mask) == 0);
+
 	ret = pages_map(NULL, size);
 	if (ret == NULL)
 		return (NULL);
-
 	offset = ALIGNMENT_ADDR2OFFSET(ret, alignment);
 	if (offset != 0) {
-#ifdef _WIN32
+		pages_unmap(ret, size);
 		return (chunk_alloc_mmap_slow(size, alignment, zero));
-#else
-		/* Try to extend chunk boundary. */
-		if (pages_map((void *)((uintptr_t)ret + size), chunksize -
-		    offset) == NULL) {
-			/*
-			 * Extension failed.  Clean up, then fall back to the
-			 * reliable-but-expensive method.
-			 */
-			pages_unmap(ret, size);
-			return (chunk_alloc_mmap_slow(size, alignment, zero));
-		} else {
-			/* Clean up unneeded leading space. */
-			pages_unmap(ret, chunksize - offset);
-			ret = (void *)((uintptr_t)ret + (chunksize - offset));
-		}
-#endif
 	}
 
 	assert(ret != NULL);