mm/hugetlb_vmemmap: fix memory loads ordering

mm/hugetlb_vmemmap: fix memory loads ordering

Using x86_64 as an example, for a 32KB struct page[] area describing a 2MB
hugeTLB, HVO reduces the area to 4KB by the following steps:

1. Split the (r/w vmemmap) PMD mapping the area into 512 (r/w) PTEs;
2. For the 8 PTEs mapping the area, remap PTE 1-7 to the page mapped
   by PTE 0, and at the same time change the permission from r/w to
   r/o;
3. Free the pages PTE 1-7 used to map, hence the reduction from 32KB
   to 4KB.

However, the following race can happen due to improperly memory loads
ordering:
  CPU 1 (HVO)                     CPU 2 (speculative PFN walker)

  page_ref_freeze()
  synchronize_rcu()
                                  rcu_read_lock()
                                  page_is_fake_head() is false
  vmemmap_remap_pte()
  XXX: struct page[] becomes r/o

  page_ref_unfreeze()
                                  page_ref_count() is not zero

                                  atomic_add_unless(&page->_refcount)
                                  XXX: try to modify r/o struct page[]

Specifically, page_is_fake_head() must be ordered after page_ref_count()
on CPU 2 so that it can only return true for this case, to avoid the later
attempt to modify r/o struct page[].

This patch adds the missing memory barrier and makes the tests on
page_is_fake_head() and page_ref_count() done in the proper order.

Link: https://lkml.kernel.org/r/20250108074822.722696-1-yuzhao@google.com
Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reported-by: Will Deacon <will@kernel.org>
Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>