Re: [Xen-devel] [PATCH 2/2] libxl: xl mem-set should not enforce memory limits

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Mar 15, 2013 at 08:01:00PM +0000, Ian Campbell wrote:
> On Fri, 2013-03-15 at 17:26 +0000, Konrad Rzeszutek Wilk wrote:
> > On Fri, Mar 15, 2013 at 05:09:51PM +0000, Ian Jackson wrote:
> > > Daniel Kiper writes ("Re: [PATCH 2/2] libxl: xl mem-set should not 
> > > enforce memory limits"):
> > > > I think that xl mem-max should be used to enforce limits. If admin
> > > > would like to enforce "hard" limit it should call xl mem-set and
> > > > xl mem-max in sequence. If we would like to leave old xl mem-set
> > > > behavior we should change comment for this command because now
> > > > it does not mention anythig about limit enforcement. Or we should
> > > > add an option which explicitly disables memory limit enforment
> > > > (this behavior is in line with xm mem-set behavior).
> > > 
> > > I think this conversation is related to the fact that at Oracle you
> > > have a different model of the Xen memory allocation model to everyone
> > > else.
> > 
> > Daniel is trying to fix an bug that Linux kernel is tripping over
> > b/c of this. Look at the converstation and patch that Daniel posted
> > a week ago for the Linux kernel.
> 
> It would be useful to mention (or at least) the rationale for a change
> such as this in the commit message.
> 
> However that's rather moot in this case because the rationale is surely
> wrong. Linux (and indeed balloon drivers generally) are expected to
> behave correctly whether the toolstack chooses to be enforcing or
> non-enforcing regarding the balloon target. So you can't "fix" the
> kernel by simply mandating that all toolstacks are non-enforcing, sorry.
> 
> > > Outside Oracle, guests are supposed to aim for the balloon target and
> > > are not permitted to exceed it (when ballooning up) or to regress
> > > (when ballooning down).
> > 
> > s/Oracle/Xend/. As Xend had this distinction. 'xm mem-set' would only set
> > the target. 'xm mem-max' on the other hand would enforce the limit.
> > 
> > Daniel is just bringing this behavior to 'xl'.
> 
> xl deliberately deviated from xend on this point.

This was :

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1273819996 -3600
# Node ID b29e42cb4727d718025b4b7039b35a824c8a11d3
# Parent  9b9a277d8a51de049068d3251b5d84b73a24196b
libxl: Adjustments to memset/memmax handling

I think xl memset should change the memory currently used by the guest
and xl memmax should change the size of the guest's address space and
not the population.  For this reason libxl_set_memory_target should
provide a way to enforce the memory target, calling
xc_domain_setmaxmem.  On the other hand xl memmax shouldn't call
xc_domain_setmaxmem because that is the upper bound of the memory
reservation, it should just change static-max, that at the moment
wouldn't do much, but we can imagine that in the future could trigger
something useful in the guest.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Acked-by: Jonathan Knowles <Jonathan.Knowles@xxxxxxxxxxxxx>=20

which just says: "I think". Was there some more background behind this?

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