Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk() is safe to walk

John Hubbard
2022-12-08T00:16:33Z

On 12/7/22 12:31, Peter Xu wrote:
> Taking vma lock here is not needed for now because all potential hugetlb
> walkers here should have i_mmap_rwsem held. Document the fact.
>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> mm/page_vma_mapped.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index e97b2e23bd28..2e59a0419d22 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -168,8 +168,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> /* The only possible mapping was handled on last iteration */
> if (pvmw->pte)
> return not_found(pvmw);
> -
> - /* when pud is not present, pte will be NULL */
> + /*
> + * NOTE: we don't need explicit lock here to walk the
> + * hugetlb pgtable because either (1) potential callers of
> + * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the
> + * caller will not walk a hugetlb vma (e.g. ksm or uprobe).
> + * When one day this rule breaks, one will get a warning
> + * in hugetlb_walk(), and then we'll figure out what to do.
> + */

Confused. Is this documentation actually intended to refer to hugetlb_walk()
itself, or just this call site? If the former, then let's move it over
to be right before hugetlb_walk().

> pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
> if (!pvmw->pte)
> return false;

thanks,
--
John Hubbard
NVIDIA