[Qemu-devel] [PULL 04/15] oslib: rework anonimous RAM allocation

["Michael S. Tsirkin" <mst@redhat.com>]

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>
---
 util/oslib-posix.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 3ae4987..27972d4 100644
--- 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;
 
-- 
MST