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

[Jürgen Groß <jgross@xxxxxxxx>]

On 17.01.20 12:36, Jan Beulich wrote:
> On 17.01.2020 12:31, Jürgen Groß wrote:
> > 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.
> All understood. Yet the change is then not a restore of prior behavior
> (just in a limited case), but a change to behavior that we never there
> before. I.e. it was indeed my assumption that the code was right, but
> the description was misleading.
The description is misleading as it fails to mention commit
96edd61dcf44362d3e, which introduced target_diff. I'll add that to
the commit message.


Juergen

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