util/mmap-alloc: Factor out reserving of a memory region to mmap_reserve()

We want to reserve a memory region without actually populating memory.
Let's factor that out.

Reviewed-by: Igor Kotrasinski <i.kotrasinsk@partner.samsung.com>
Acked-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine core
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210510114328.21835-3-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/mmap-alloc.c

@@ -82,6 +82,38 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
     return qemu_real_host_page_size;
 }
 
+/*
+ * Reserve a new memory region of the requested size to be used for mapping
+ * from the given fd (if any).
+ */
+static void *mmap_reserve(size_t size, int fd)
+{
+    int flags = MAP_PRIVATE;
+
+#if defined(__powerpc64__) && defined(__linux__)
+    /*
+     * On ppc64 mappings in the same segment (aka slice) must share the same
+     * page size. Since we will be re-allocating part of this segment
+     * from the supplied fd, we should make sure to use the same page size, to
+     * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
+     * avoid allocating backing store memory.
+     * We do this unless we are using the system page size, in which case
+     * anonymous memory is OK.
+     */
+    if (fd == -1 || qemu_fd_getpagesize(fd) == qemu_real_host_page_size) {
+        fd = -1;
+        flags |= MAP_ANONYMOUS;
+    } else {
+        flags |= MAP_NORESERVE;
+    }
+#else
+    fd = -1;
+    flags |= MAP_ANONYMOUS;
+#endif
+
+    return mmap(0, size, PROT_NONE, flags, fd, 0);
+}
+
 static inline size_t mmap_guard_pagesize(int fd)
 {
 #if defined(__powerpc64__) && defined(__linux__)
@@ -104,7 +136,6 @@ void *qemu_ram_mmap(int fd,
     int prot;
     int flags;
     int map_sync_flags = 0;
-    int guardfd;
     size_t offset;
     size_t total;
     void *guardptr;
@@ -116,30 +147,7 @@ void *qemu_ram_mmap(int fd,
      */
     total = size + align;
 
-#if defined(__powerpc64__) && defined(__linux__)
-    /* On ppc64 mappings in the same segment (aka slice) must share the same
-     * page size. Since we will be re-allocating part of this segment
-     * from the supplied fd, we should make sure to use the same page size, to
-     * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
-     * avoid allocating backing store memory.
-     * We do this unless we are using the system page size, in which case
-     * anonymous memory is OK.
-     */
-    flags = MAP_PRIVATE;
-    if (fd == -1 || guard_pagesize == qemu_real_host_page_size) {
-        guardfd = -1;
-        flags |= MAP_ANONYMOUS;
-    } else {
-        guardfd = fd;
-        flags |= MAP_NORESERVE;
-    }
-#else
-    guardfd = -1;
-    flags = MAP_PRIVATE | MAP_ANONYMOUS;
-#endif
-
-    guardptr = mmap(0, total, PROT_NONE, flags, guardfd, 0);
-
+    guardptr = mmap_reserve(total, fd);
     if (guardptr == MAP_FAILED) {
         return MAP_FAILED;
     }