Re: [PATCH v3] writeback: fix obtain a reference to a freeing memcg css

[Muchun Song <songmuchun@bytedance.com>]

Hi,

It seems like this patch has not been added to the linux-next
tree. Can anyone help with this? Thanks.

On Fri, Apr 2, 2021 at 5:13 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> The caller of wb_get_create() should pin the memcg, because
> wb_get_create() relies on this guarantee. The rcu read lock
> only can guarantee that the memcg css returned by css_from_id()
> cannot be released, but the reference of the memcg can be zero.
>
>   rcu_read_lock()
>   memcg_css = css_from_id()
>   wb_get_create(memcg_css)
>       cgwb_create(memcg_css)
>           // css_get can change the ref counter from 0 back to 1
>           css_get(memcg_css)
>   rcu_read_unlock()
>
> Fix it by holding a reference to the css before calling
> wb_get_create(). This is not a problem I encountered in the
> real world. Just the result of a code review.
>
> Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
> Changelog in v3:
>  1. Do not change GFP_ATOMIC.
>  2. Update commit log.
>
>  Thanks for Michal's review and suggestions.
>
> Changelog in v2:
>  1. Replace GFP_ATOMIC with GFP_NOIO suggested by Matthew.
>
>
>  fs/fs-writeback.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 3ac002561327..dedde99da40d 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -506,9 +506,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>         /* find and pin the new wb */
>         rcu_read_lock();
>         memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
> -       if (memcg_css)
> -               isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> +       if (memcg_css && !css_tryget(memcg_css))
> +               memcg_css = NULL;
>         rcu_read_unlock();
> +       if (!memcg_css)
> +               goto out_free;
> +
> +       isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> +       css_put(memcg_css);
>         if (!isw->new_wb)
>                 goto out_free;
>
> --
> 2.11.0
>