Re: [PATCH RFC 02/10] mm/hugetlb: Comment huge_pte_offset() for its locking requirements

Nadav Amit
2022-11-01T05:46:57Z

On Oct 30, 2022, at 2:29 PM, Peter Xu <peterx@redhat.com> wrote:

> huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a
> hugetlb address.
>
> Normally, it's always safe to walk the 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: hugetlbfs has the pmd sharing feature, it
> means that even with mmap lock held, the PUD pgtable page can still go away
> from under us if pmd unsharing is possible during the walk.
>
> It's not always the case, e.g.:
>
> (1) If the mapping is private we're not prone to pmd sharing or
> unsharing, so it's okay.
>
> (2) If we're with the hugetlb vma lock held for either read/write, it's
> okay too because pmd unshare cannot happen at all.
>
> Document all these explicitly for huge_pte_offset(), because it's really
> not that obvious. This also tells all the callers on what it needs to
> guarantee huge_pte_offset() thread-safety.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> arch/arm64/mm/hugetlbpage.c | 32 ++++++++++++++++++++++++++++++++

Please excuse my ignorant question - is there something specific for arm64
code here? Other archs seem to have similar code, no?