[Xen-devel] Balloon driver bug in increase_reservation

[Wei Liu <wei.liu2@xxxxxxxxxx>]

Hi, Stefano

I found another bug in the balloon scratch page code. As I didn't follow
the discussion on scratch page so I cannot propose a proper fix at the
moment.

The problem is that in balloon.c:increase_reservation, when a ballooned
page is resued, it can have a valid P2M entry pointing to the scratch,
hitting the BUG_ON

   BUG_ON(!xen_feature(XENFEAT_auto_translated_physmap) &&
          phys_to_machine_mapping_valid(pfn));

As balloon worker might run by a CPU other then the one that returns the
page, checking pfn_to_mfn(pfn) == local_cpu_scratch_page_mfn wouldn't
work.  Checking pfn_to_mfn(pfn) belongs to the set of all scratch page
mfns is not desirable.

My thoughts so far:
 1. remove that BUG_ON (looks like a wrong fix though)
 2. make balloon scratch page global

Other thoughts?

Wei.

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