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

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

On Tue, Sep 12, 2017 at 09:19:23PM -0400, Boris Ostrovsky wrote:
> 
> 
> 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?

No. I just cold-booted it and tried to livepatch.
> 
> Another thing to try is to set need_scrub to true in free_heap_pages().

Magic!

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index dbad1e1ca0..9303eb4517 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1308,6 +1308,7 @@ static void free_heap_pages(
     ASSERT(node >= 0);
 
     spin_lock(&heap_lock);
+    need_scrub = true;
 
     for ( i = 0; i < (1 << order); i++ )
     {

Fixes it ! :-)


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

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