[Xen-devel] An issue in xen_limit_pages_to_max_mfn() in Xenlinux Ver. 2.6.18

[Daniel Kiper <dkiper@xxxxxxxxxxxx>]

Hi Jan,

During work on kexec/kdump for Xen domU I found that
xen_limit_pages_to_max_mfn() registers undo_limit_pages()
destructor which breaks __free_pages(). When __free_pages()
is called then at beginning of this function put_page_testzero()
is called which decrements page count for given page. Later
undo_limit_pages() destructor is called which once again
calls __free_pages() and in consequence put_page_testzero()
fails (BUG_ON() is called) because page count is 0. It could
be easily fixed, however, after reviewing xen_limit_pages_to_max_mfn()
I could not find any good reason for which undo_limit_pages()
destructor is registered. Maybe it could be removed at all because
all pages are freed when __free_pages() is called and in this
case we do not care where they live. However, maybe I missed
something important. Could you explain for which cases
undo_limit_pages() destructor was prepared ???

Daniel

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