Re: [PATCH v2] xen/page_alloc: Simplify domain_adjust_tot_pages

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Mar 11, 2025 at 02:53:04PM +0000, Alejandro Vallejo wrote:
> On Tue Mar 11, 2025 at 12:46 PM GMT, Roger Pau Monné wrote:
> > On Tue, Mar 04, 2025 at 11:10:00AM +0000, Alejandro Vallejo wrote:
> > > The logic has too many levels of indirection and it's very hard to
> > > understand it its current form. Split it between the corner case where
> > > the adjustment is bigger than the current claim and the rest to avoid 5
> > > auxiliary variables.
> > > 
> > > Add a functional change to prevent negative adjustments from
> > > re-increasing the claim. This has the nice side effect of avoiding
> > > taking the heap lock here on every free.
> > > 
> > > While at it, fix incorrect field name in nearby comment.
> > > 
> > > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> >
> > Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Thanks.
> 
> > I think it would be nice to also ensure that once the domain is
> > finished building (maybe when it's unpaused for the first
> > time?) d->outstanding_pages is set to 0.  IMO the claim system was
> > designed to avoid races during domain building, and shouldn't be used
> > once the domain is already running.
> >
> > Thanks, Roger.
> 
> As a matter of implementation that's already the case by toolstack being 
> "nice"
> and unconditionally clearing claims after populating the physmap.

I see.  Another option would be to refuse the unpause a domain if it
still has pending claims.  However I don't know how that will work out
with all possible toolstacks.

> However, I agree the hypervisor should do it on its own. I didn't find a
> suitable place for it. 

You could do it in arch_domain_creation_finished().

> It may be possible to do it on every unpause with
> minimal overhead because it doesn't need to take the heap lock unless there 
> are
> outstanding claims on the domains. Would that sound like an ok idea? I'd 
> rather
> not add even more state into "struct domain" to count pauses...

There's another side missing IMO, XENMEM_claim_pages should return an
error (and refuse the set any claims) once d->creation_finished ==
true.

Thanks, Roger.