exec: always use MADV_DONTFORK

MADV_DONTFORK prevents fork to fail with -ENOMEM if the default
overcommit heuristics decides there's too much anonymous virtual
memory allocated. If the KVM secondary MMU is synchronized with MMU
notifiers or not, doesn't make a difference in that regard.

Secondly it's always more efficient to avoid copying the guest
physical address space in the fork child (so we avoid to mark all the
guest memory readonly in the parent and so we skip the establishment
and teardown of lots of pagetables in the child).

In the common case we can ignore the error if MADV_DONTFORK is not
available. Leave a second invocation that errors out in the KVM path
if MMU notifiers are missing and KVM is enabled, to abort in such
case.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Tested-By: Benoit Canet <benoit@irqsave.net>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

diff --git a/exec.c b/exec.c
index 030118e68edc8d4d1f83f46eaae032c87fe1d009..5e6015cbd3af575342988ccda5e6c26fb1754b6d 100644 (file)
--- a/exec.c
+++ b/exec.c
@@ -1157,6 +1157,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
 
     qemu_ram_setup_dump(new_block->host, size);
     qemu_madvise(new_block->host, size, QEMU_MADV_HUGEPAGE);
+    qemu_madvise(new_block->host, size, QEMU_MADV_DONTFORK);
 
     if (kvm_enabled())
         kvm_setup_guest_memory(new_block->host, size);