Re: [PATCH RESEND v6 02/16] mm/gup: Fix __get_user_pages() on fault retry of hugetlb

[David Hildenbrand <david@redhat.com>]

On 20.02.20 16:53, Peter Xu wrote:
> When follow_hugetlb_page() returns with *locked==0, it means we've got
> a VM_FAULT_RETRY within the fauling process and we've released the
> mmap_sem.  When that happens, we should stop and bail out.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/gup.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 1b4411bd0042..76cb420c0fb7 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -849,6 +849,16 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  				i = follow_hugetlb_page(mm, vma, pages, vmas,
>  						&start, &nr_pages, i,
>  						gup_flags, locked);
> +				if (locked && *locked == 0) {
> +					/*
> +					 * We've got a VM_FAULT_RETRY
> +					 * and we've lost mmap_sem.
> +					 * We must stop here.
> +					 */
> +					BUG_ON(gup_flags & FOLL_NOWAIT);
> +					BUG_ON(ret != 0);

Can we be sure ret is really set to != 0 at this point? At least,
reading the code this is not clear to me.

Shouldn't we set "ret = i" and assert that i is an error (e.g., EBUSY?).
Or set -EBUSY explicitly?

-- 
Thanks,

David / dhildenb