Re: [Xen-devel] CONFIG_SCRUB_DEBUG=y + arm64 + livepatch = Xen BUG at page_alloc.c:738

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 09/12/2017 08:01 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Sep 11, 2017 at 08:45:02PM -0400, Boris Ostrovsky wrote:
> > On 09/11/2017 07:55 PM, Konrad Rzeszutek Wilk wrote:
> > > Hey,
> > >
> > > I've only been able to reproduce this on ARM64 (trying right now ARM32
> > > as well), and not on x86.
> > >
> > > If I compile Xen without CONFIG_SCRUB_DEBUG it works great. But if
> > > enable it and try to load a livepatch it blows up in page_alloc.c:738
> > >
> > > This is with origin/staging (d0291f3391)
> > Can you still reproduce this if you revert 307c3be?
> Sadly yes - it still crashes. I didn't capture the serial output.
>
> I honestly think the issue is that on ARM64 the "sleep" loop does not
> wake up as often as on x86 (CC-ing Dariof who I believe observed this
> with Credit2 and the wakeup.. something) - maybe he remembers the
> details. Anyhow my theory is that the pages are not scrubbed at all
> when they go in the idle loop as once it goes to sleep - it stays there.

There is no (well, should not be) any timing dependencies in how/whether 
pages are scrubbed. If a page doesn't get scrubbed because someone 
didn't wake up then it should be scrubbed in alloc_heap_pages(). So in 
this case the page is thought to be clean (_PGC_need_scrub is not set), 
but it is not.
Have you tried running a guest (or two), rebooting in a loop?

Another thing to try is to set need_scrub to true in free_heap_pages().

-boris


> Ah, see commit 05c52278a7c92bc753d9fe32017e4961012b9f23
>
> Maybe this is related?
> > -boris
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