Re: [PATCH] xen/balloon: Support xend-based toolstack take two

[JÃrgen GroÃ]

On 17.01.20 12:01, Jan Beulich wrote:
> On 16.01.2020 18:00, Juergen Gross wrote:
> > Commit 3aa6c19d2f38be ("xen/balloon: Support xend-based toolstack")
> > tried to fix a regression with running on rather ancient Xen versions.
> > Unfortunately the fix was based on the assumption that xend would
> > just use another Xenstore node, but in reality only some downstream
> > versions of xend are doing that. The upstream xend does not write
> > that Xenstore node at all, so the problem must be fixed in another
> > way.
> >
> > The easiest way to achieve that is to fall back to the behavior before
> > commit 5266b8e4445c ("xen: fix booting ballooned down hvm guest")
> > in case the static memory maximum can't be read.
>
> I could use some help here: Prior to said commit there was
>
> 	target_diff = new_target - balloon_stats.target_pages;
>
>
> Which is, afaict, ...
>
> > --- a/drivers/xen/xen-balloon.c
> > +++ b/drivers/xen/xen-balloon.c
> > @@ -94,7 +94,7 @@ static void watch_target(struct xenbus_watch *watch,
> >   				  "%llu", &static_max) == 1))
> >   			static_max >>= PAGE_SHIFT - 10;
> >   		else
> > -			static_max = new_target;
> > +			static_max = balloon_stats.current_pages;
> >   
> >   		target_diff = (xen_pv_domain() || xen_initial_domain()) ? 0
> >   				: static_max - balloon_stats.target_pages;
>
> ... what the code does before your change. Afaict there was
> never a use of balloon_stats.current_pages in this function.

That is a little bit indirect, yes. In the end I want static_max to
be either the maximum reported by Xen, or if not available, the current
assumed memory size, which can be found in balloon_stats.current_pages.

The main idea is to avoid a negative target_diff which would result in
not ballooning down.


Juergen