mm/hugetlb: document huge_pte_offset usage
huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a hugetlb address. Normally, it's always safe to walk a generic pgtable as long as we're with the mmap lock held for either read or write, because that guarantees the pgtable pages will always be valid during the process. But it's not true for hugetlbfs, especially shared: hugetlbfs can have its pgtable freed by pmd unsharing, it means that even with mmap lock held for current mm, the PMD pgtable page can still go away from under us if pmd unsharing is possible during the walk. So we have two ways to make it safe even for a shared mapping: (1) If we're with the hugetlb vma lock held for either read/write, it's okay because pmd unshare cannot happen at all. (2) If we're with the i_mmap_rwsem lock held for either read/write, it's okay because even if pmd unshare can happen, the pgtable page cannot be freed from under us. Document it. Link: https://lkml.kernel.org/r/20221216155100.2043537-4-peterx@redhat.comSigned-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: John Hubbard <jhubbard@nvidia.com> Reviewed-by: David Hildenbrand <david@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: James Houghton <jthoughton@google.com> Cc: Jann Horn <jannh@google.com> Cc: Miaohe Lin <linmiaohe@huawei.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Muchun Song <songmuchun@bytedance.com> Cc: Nadav Amit <nadav.amit@gmail.com> Cc: Rik van Riel <riel@surriel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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