Re: [PATCH v2 1/2] secretmem/gup: don't check if page is secretmem without reference

David Hildenbrand
2021-04-20T13:20:11Z

On 20.04.21 15:16, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
>
> The check in gup_pte_range() whether a page belongs to a secretmem mapping
> is performed before grabbing the page reference.
>
> To avoid potential race move the check after try_grab_compound_head().
>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
> mm/gup.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index c3a17b189064..4b58c016e949 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2080,13 +2080,13 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
> page = pte_page(pte);
>
> - if (page_is_secretmem(page))
> - goto pte_unmap;
> -
> head = try_grab_compound_head(page, 1, flags);
> if (!head)
> goto pte_unmap;
>
> + if (page_is_secretmem(page))
> + goto pte_unmap;
> +

Looking at the hunk below, I wonder if you're missing a put_compound_head().

(also, I'd do if unlikely(page_is_secretmem()) but that's a different
discussion)

> if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> put_compound_head(head, 1, flags);
> goto pte_unmap;
>

--
Thanks,

David / dhildenb