On Oct 30, 2022, at 2:29 PM, Peter Xu <peterx@xxxxxxxxxx> 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@xxxxxxxxxx> > --- > 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?
