Re: [Qemu-devel] PING^2 Re: [PATCH] KVM: always use MADV_DONTFORK

["Andreas Färber" <afaerber@suse.de>]

Am 30.08.2013 17:48, schrieb Paolo Bonzini:
> Il 06/08/2013 18:47, Paolo Bonzini ha scritto:
>> On 07/25/2013 12:11 PM, Andrea Arcangeli wrote:
>>> 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>
>>> ---
>>>   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);
>>>
>>>
>>
>> PING.
>>
>> Benoit reported this on IRC, too.
>>
>> Paolo
>>
>>
> 
> PING^2
> 
> The last paragraph of the commit message should answer Andreas's objection.

OK. The patch is mislabelled as "KVM:" though putting this in a common
code path and touching exec.c only, so please put it on uq/master and
fix that up before you set the history in stone. :)

Andreas

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