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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Thu, Jan 9, 2020 at 8:34 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> 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()?
>

My reasoning was that by default the user should pause the parent VM
before forking. This sanity checks just mimicks that step in case the
user didn't do that already. But I guess either would work, I don't
really see much difference between the two.

Tamas

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