Re: [Xen-devel] [PATCH v4 15/18] xen/mem_sharing: VM forking

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.01.2020 16:10, Roger Pau Monné wrote:
> On Thu, Jan 09, 2020 at 06:41:12AM -0700, Tamas K Lengyel wrote:
>> On Thu, Jan 9, 2020 at 3:29 AM Julien Grall <julien@xxxxxxx> wrote:
>>>
>>> Hi Tamas,
>>>
>>> On 08/01/2020 17:14, Tamas K Lengyel wrote:
>>>> +static int mem_sharing_fork(struct domain *d, struct domain *cd)
>>>> +{
>>>> +    int rc;
>>>> +
>>>> +    if ( !d->controller_pause_count &&
>>>> +         (rc = domain_pause_by_systemcontroller(d)) )
>>>
>>> AFAIU, the parent domain will be paused if it wasn't paused before and
>>> this will not be unpaused by the same hypercall. Right?
>>
>> Yes, it needs to remain paused as long as there are forks active from
>> it. Afterwards it can be unpaused.
> 
> If you want the parent domain to remain paused for as long as the
> forks are active, shouldn't each fork increment the pause count on
> creation and decrement it when the fork is destroyed?
> 
> How can you assure no other operation or entity has incremented
> controller_pause_count temporary and is likely to decrement it at some
> point while forks are still active?

The _by_systemcontroller variants look wrong to be used here anyway.
Why is this not simply domain_{,un}pause()?

Jan

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