[Xen-devel] Linux: removal of contiguous_bitmap broke kexec

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

Since kexec uses xen_destroy_contiguous_region() on memory it is about
to free, but passed this through xen_limit_pages_to_max_mfn() only, the
contiguous_bitmap check in the old code prevented any action to be taken
on such regions. This could therefore be fixed in at least three different
ways:

1) Since the call to xen_destroy_contiguous_region() really was pointless
before, we could just eliminate that from kernel/kexec.c.

2) xen_destroy_contiguous_region() could check the region it is passed
and bail out if it turns out not to be contiguous.

3) We could track which regions were actually acted upon by
xen_limit_pages_to_max_mfn() (through PageForeign logic) and attempt
to undo the limitation in the destructor (and at once again eliminate the
[mismatched] call to xen_destroy_contiguous_region()). Since this case
is not performance critical, there shouldn't be an issue with not batching
these. And a check of the kexec code indicates it doesn't use the index
member of struct page.

Opinions appreciated. Thanks,
Jan


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