it doesn't (of course depends "what's the right thing"), and that's why
Manfred is asking after I asked him if he was really sure the API was
the right one, but as said to him, something is wrong with the kernel
too somehow, either we remove mark_page_present from find_extend_vma, or
we add it to the page fault handler too.
What the current kernel is doing with page faults, is to fault in only
the touched pages, not the pages in between as well, this isn't a
security concern because the faulted in pages won't be swapped out, but
it may matter for some RT app, OTOH the RT apps would better memset the
whole stack they need before assuming they won't get page faults, first
of all because of all other kernels out there (this is what I mean with
a matter of API).
I guess it is cleaner if a VM_LOCKED vma has all its pages allocated
between vm_start and vm_end, so I guess adding the mark_page_present in
do_page_fault as suggested by Manfred is ok.
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