Xen devel - Nov 2008 - Linux: removal of contiguous_bitmap broke kexec

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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(1) is good for now, and I prefer it to (2). Of course it would be nice to
have the ability to exchange the low-memory pages for ordinary pages once
kexec is done with them, but it can be done as a separate patch. I''ll
do (1)
right now, since it is trivial.

 -- Keir

On 25/11/08 11:04, "Jan Beulich" <jbeulich@novell.com> wrote:
> 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
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel


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