Re: [Qemu-devel] [PATCH] KVM: always use MADV_DONTFORK

[Benoît Canet]

Le Thursday 25 Jul 2013 à 12:11:15 (+0200), Andrea Arcangeli a écrit :
> 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 <address@hidden>
> ---
>  exec.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/exec.c b/exec.c
> index c99a883..d3bb58d 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1162,6 +1162,7 @@ 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);
> 

This patch solve a bug where the network down script of a regular tap interface
is not executed because fork fail when a pci-assigment is done on a large guest.

Tested-By: Benoit Canet <address@hidden>