Re: xenstored: Interaction bettwen SET_PERMS and transaction

[Julien Grall <julien@xxxxxxx>]

Hi Juergen,

On 24/02/2023 16:00, Juergen Gross wrote:
> On 24.02.23 16:00, Julien Grall wrote:
> > Hi Juergen,
> >
> > For some internal purpose, I need to write a script that would do the 
> > following:
> >    1) Start a transaction
> >    2) Call GET_PERMS
> >    3) Call SET_PERMS with the permission just retrieved
> >    4) Commit the transaction
> >
> > (Don't ask why :))
> >
> > This was executed from dom0 on every nodes. In some cases, we noticed 
> > that 4) would return -ENOSPC.
> > When digging through the code, it looks like this is because 
> > transaction_fix_domains() (now called acc_fix_domains()) will check 
> > that the quota is correct.
> > The domain is question were over its limit, but given this is called 
> > by dom0, I would have expected that it should not never return -ENOSPC 
> > (note that a SET_PERMS outside of a transaction would work, but this 
> > can't be used our case).
> > Furthermore, the transaction is not changing any accounting. So I find 
> > a bit strange that we would prevent it even if this was run from an 
> > unprivileged domain.
> > Do you know if this issues would be fixed by your current rework?
> >
> > If not, then I think we at least want to skip the quota check if the 
> > domain is privileged.
> > For non-privileged domain, I am not entirely sure what to do. I was 
> > originally thinking to check if cd->nbentry is 0 and then skip the 
> > quota check. But this would allow a domain to remove a node and then 
> > replace by a new one (I think we still want to forbid that).
> > What do you think?
> Patch sent. Could you please test it with your script?
Thanks! I will give a try next week and let you know the result.

Cheers,

--
Julien Grall