oslib: rework anonimous RAM allocation

At the moment we first allocate RAM, sometimes more than necessary for
alignment reasons.  We then free the extra RAM.

Rework this to avoid the temporary allocation: reserve the
range by mapping it with PROT_NONE, then use just the
necessary range with MAP_FIXED.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 3ae4987b6bf9a9a3c38ed7754dc0c3a9fda1a050..27972d4addc67d533e200de92a1c61836ee9f1d1 100644 (file)
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -129,9 +129,9 @@ void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment)
 {
     size_t align = QEMU_VMALLOC_ALIGN;
     size_t total = size + align - getpagesize();
-    void *ptr = mmap(0, total, PROT_READ | PROT_WRITE,
-                     MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+    void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
     size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
+    void *ptr1;
 
     if (ptr == MAP_FAILED) {
         return NULL;
@@ -140,6 +140,14 @@ void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment)
     if (alignment) {
         *alignment = align;
     }
+
+    ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
+                MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+    if (ptr1 == MAP_FAILED) {
+        munmap(ptr, total);
+        return NULL;
+    }
+
     ptr += offset;
     total -= offset;