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From 38dcbd1ff68da81b9172d8036622e41fe591c5c0 Mon Sep 17 00:00:00 2001
From: Andrea Arcangeli <>
Date: Fri, 7 Feb 2014 11:02:33 +0100
Subject: [PATCH 10/28] exec: always use MADV_DONTFORK

RH-Author: Andrea Arcangeli <>
Message-id: <>
Patchwork-id: 57131
O-Subject: [RHEL-7.0 qemu-kvm PATCH] exec: always use MADV_DONTFORK
Bugzilla: 1004197
RH-Acked-by: Paolo Bonzini <>
RH-Acked-by: Andrew Jones <>
RH-Acked-by: Dr. David Alan Gilbert (git) <>

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

Signed-off-by: Andrea Arcangeli <>
Tested-By: Benoit Canet <>
Acked-by: Paolo Bonzini <>
Signed-off-by: Gleb Natapov <>
(cherry picked from upstream commit 3e469dbfe413c25d48321c3a19ddfae0727dc6e5)
 exec.c | 1 +
 1 file changed, 1 insertion(+)

Signed-off-by: Miroslav Rezanina <>
 exec.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/exec.c b/exec.c
index ce9310c..6fe2fe7 100644
--- a/exec.c
+++ b/exec.c
@@ -1194,6 +1194,7 @@ 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);