[Xen-devel] RE: [PATCH v2] xen: Fix selfballooning and ensure it doesn't go too far

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: David Vrabel [mailto:david.vrabel@xxxxxxxxxx]
> Cc: Konrad Wilk; linux-kernel@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxx; 
> Jeremy Fitzhardinge
> Subject: Re: [PATCH v2] xen: Fix selfballooning and ensure it doesn't go too 
> far
> 
> On 27/09/11 17:19, Dan Magenheimer wrote:
> >> From: David Vrabel [mailto:david.vrabel@xxxxxxxxxx]
> >> Subject: Re: [PATCH v2] xen: Fix selfballooning and ensure it doesn't go 
> >> too far
> >>
> >> On 27/09/11 16:03, Dan Magenheimer wrote:
> >>> Note: This patch is also now in a git tree at:
> >>>
> >>> git://oss.oracle.com/git/djm/tmem.git#selfballoon-fix-v2
> >>>
> >>> The balloon driver's "current_pages" is very different from
> >>> totalram_pages.  Self-ballooning needs to be driven by
> >>> the latter.
> >
> > Hi David --
> >
> > Thanks for the feedback!
> >
> >> I don't think this part of the change makes any difference. It looks like 
> >> it
> >> rearranges the maths without changing the end result (other than
> >> slightly increasing the rate of change).
> >> I think this (partial, untested) patch is equivalent:
> >
> > Actually it does.
> 
> Really?
> 
> Both patched and unpatched the new target, S, is (eventually):
> 
>    S = V + F + C - T
> 
> where V is vm_committed_as, F is frontswap_curr_pages(), C is
> balloon_stats.current_pages, and T = totalram_pages.

Sorry, in my haste to shoot off a quick reply while my mind
was somewhere else, I see my reply was poor and misleading.

Yes, "S", the value passed to balloon_set_new_target(), is
the same in most cases.  However, it is V+F, not S, that must
be compared against M (= the floor function); the "target"
of selfballooning, the value that the kernel cares about
(not the value that Xen cares about) is max(V+F,M).  This
gets converted to Xen-cares-about, IOW:

        S = max(V+F,M) + C - T

where S is passed to balloon_set_new_target.

> Perhaps the refactoring of the maths is a good idea (I don't think so)
> but it shouldn't be part of this patch and it shouldn't be described as
> a fix.

The refactored version makes sense now from a kernel perspective,
though I can see how it might be confusing from a Xen perspective,
especially to a balloon driver expert such as yourself.

It is most definitely a fix because the formula is different
and OOMs that previously happened no longer happen.  I don't
think the commit comment describes the *refactoring* as a fix,
just says that self-ballooning needs to be driven by kernel-
cares-about values (even if it has to interface to the
Xen balloon driver with a Xen-cares-about parameter).

Hopefully that makes more sense?

Dan

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