Re: [PATCH v4] xen/balloon: add late_initcall_sync() for initial ballooning done

[Juergen Gross <jgross@xxxxxxxx>]

On 04.11.21 17:34, Boris Ostrovsky wrote:
> On 11/4/21 12:21 PM, Juergen Gross wrote:
> > On 04.11.21 16:55, Boris Ostrovsky wrote:
> > > On 11/3/21 9:55 PM, Boris Ostrovsky wrote:
> > > > On 11/2/21 5:19 AM, Juergen Gross wrote:
> > > > > When running as PVH or HVM guest with actual memory < max memory the
> > > > > hypervisor is using "populate on demand" in order to allow the guest
> > > > > to balloon down from its maximum memory size. For this to work
> > > > > correctly the guest must not touch more memory pages than its target
> > > > > memory size as otherwise the PoD cache will be exhausted and the guest
> > > > > is crashed as a result of that.
> > > > >
> > > > > In extreme cases ballooning down might not be finished today before
> > > > > the init process is started, which can consume lots of memory.
> > > > >
> > > > > In order to avoid random boot crashes in such cases, add a late init
> > > > > call to wait for ballooning down having finished for PVH/HVM guests.
> > > > >
> > > > > Warn on console if initial ballooning fails, panic() after stalling
> > > > > for more than 3 minutes per default. Add a module parameter for
> > > > > changing this timeout.
> > > > >
> > > > > Cc: <stable@xxxxxxxxxxxxxxx>
> > > > > Reported-by: Marek Marczykowski-Górecki 
> > > > > <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > >
> > > >
> > > > Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > >
> > > This appears to have noticeable effect on boot time (and boot 
> > > experience in general).
> > >
> > > I have
> > >
> > >
> > >    memory=1024
> > >    maxmem=8192
> > >
> > >
> > > And my boot time (on an admittedly slow box) went from 33 to 45 
> > > seconds. And boot pauses in the middle while it is waiting for 
> > > ballooning to complete.
> > >
> > > [    5.062714] xen:balloon: Waiting for initial ballooning down 
> > > having finished.
> > > [    5.449696] random: crng init done
> > > [   34.613050] xen:balloon: Initial ballooning down finished.
> > This shows that before it was just by chance that the PoD cache wasn't
> > exhausted.
>
> True.
>
>
> > > So at least I think we should consider bumping log level down from info.
> > Which level would you prefer? warn?
> Notice? Although that won't make much difference as WARN is the default 
> level.
Right. That was my thinking.

> I suppose we can't turn scrubbing off at this point?
I don't think we can be sure a ballooned page wasn't in use before. And
it could contain some data e.g. from the loaded initrd, maybe even put
there by the boot loader. So no, I wouldn't want to do that by default.

We could add another value to the xen_scrub_pages boot parameter, like
xen_scrub_pages=not-at-boot or some such. But this should be another
patch. And it should be documented that initrd or kernel data might
leak.

> > And if so, would you mind doing this while committing (I have one day
> > off tomorrow)?
>
> Yes, of course.
Thanks.


Juergen

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