Re: [PATCH v2] mm/z3fold.c: Reinitialize zhdr structs after migration

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jul 15, 2019 at 5:05 PM Henry Burns <henryburns@google.com> wrote:
>
> z3fold_page_migration() calls memcpy(new_zhdr, zhdr, PAGE_SIZE).
> However, zhdr contains fields that can't be directly coppied over (ex:
> list_head, a circular linked list). We only need to initialize the
> linked lists in new_zhdr, as z3fold_isolate_page() already ensures
> that these lists are empty
>
> Additionally it is possible that zhdr->work has been placed in a
> workqueue. In this case we shouldn't migrate the page, as zhdr->work
> references zhdr as opposed to new_zhdr.
>
> Fixes: bba4c5f96ce4 ("mm/z3fold.c: support page migration")
> Signed-off-by: Henry Burns <henryburns@google.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> ---
>  Changelog since v1:
>  - Made comments explicityly refer to new_zhdr->buddy.
>
>  mm/z3fold.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 42ef9955117c..f4b2283b19a3 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -1352,12 +1352,22 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
>                 z3fold_page_unlock(zhdr);
>                 return -EBUSY;
>         }
> +       if (work_pending(&zhdr->work)) {
> +               z3fold_page_unlock(zhdr);
> +               return -EAGAIN;
> +       }
>         new_zhdr = page_address(newpage);
>         memcpy(new_zhdr, zhdr, PAGE_SIZE);
>         newpage->private = page->private;
>         page->private = 0;
>         z3fold_page_unlock(zhdr);
>         spin_lock_init(&new_zhdr->page_lock);
> +       INIT_WORK(&new_zhdr->work, compact_page_work);
> +       /*
> +        * z3fold_page_isolate() ensures that new_zhdr->buddy is empty,
> +        * so we only have to reinitialize it.
> +        */
> +       INIT_LIST_HEAD(&new_zhdr->buddy);
>         new_mapping = page_mapping(page);
>         __ClearPageMovable(page);
>         ClearPagePrivate(page);
> --
> 2.22.0.510.g264f2c817a-goog
>