Re: [Xen-devel] [PATCH] x86/PAE partially linear page table support

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>That's not what I mean. Say we have two L2 tables, A and B. A[0] references
>B and B[0] references A. Even when all external references to A and B have
>gone away (i.e., there are no references from L3 tables) the type counts of
>A and B will both be non-zero because of the circular reference. This will
>prevent the pages from being freed and, ultimately, can lead to the domain
>never being destroyed.
>
>This case needs to be handled. See the code just below the largest comment
>in arch/x86/domain.c:relinquish_memory() to see how it's handled for
>top-level pagetables. Unfortunately it doesn't quite trivially generalise.

Hmm, it would seem to me that while it may not easily generalize, it at least
properly extends to the case the patch tries to address, by exchanging
PGT_base_page_table against PGT_l2_page_table. Really, in current code
this can never happen for PGT_base_page_table in PAE (or compat mode
guests), as such references aren't being established anywhere, and hence
the code should be dead for such guests.

But even for the general case - wouldn't it be possible to call
relinquish_memory() CONFIG_PAGING_LEVELS-1 times, each time lowering
the level of page tables that can be forced invalid by one?

Jan

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