On Tue, Jun 15, 2021 at 01:03:53PM +0200, Jann Horn wrote:

The messier path, as the original commit describes, is "gigantic" page
allocation. In that case, we'll go through the following path (if we
ignore CMA):

  alloc_fresh_huge_page():
    alloc_gigantic_page()
      alloc_contig_pages()
        __alloc_contig_pages()
          alloc_contig_range()
            isolate_freepages_range()
              split_map_pages()
                post_alloc_hook() [FOR EVERY PAGE]
                  set_page_refcounted()
                    set_page_count(page, 1)
    prep_compound_gigantic_page()
      set_page_count(p, 0) [FOR EVERY TAIL PAGE]

so all the tail pages are initially allocated with refcount 1 by the
page allocator, and then we overwrite those refcounts with zeroes.


Luckily, the only non-__init codepath that can get here is
__nr_hugepages_store_common(), which is only invoked from privileged
writes to sysfs/sysctls.

Argh.  What if we passed __GFP_COMP into alloc_contig_pages()?
The current callers of alloc_contig_range() do not pass __GFP_COMP,
so it's no behaviour change for them, and __GFP_COMP implies this
kind of behaviour.  I think that would imply _not_ calling
split_map_pages(), which implies not calling post_alloc_hook(),
which means we probably need to do a lot of the parts of
post_alloc_hook() in alloc_gigantic_page().  Yuck.