Re: [PATCH v2 10/28] writeback: prevent memory cgroup release in writeback module

[Johannes Weiner]

On Wed, Dec 17, 2025 at 03:27:34PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> 
> In the near future, a folio will no longer pin its corresponding
> memory cgroup. To ensure safety, it will only be appropriate to
> hold the rcu read lock or acquire a reference to the memory cgroup
> returned by folio_memcg(), thereby preventing it from being released.
> 
> In the current patch, the function get_mem_cgroup_css_from_folio()
> and the rcu read lock are employed to safeguard against the release
> of the memory cgroup.
> 
> This serves as a preparatory measure for the reparenting of the
> LRU pages.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

Looks sane to me.

The roo_mem_cgroup handling in get_mem_cgroup_css_from_folio() is
unusual - usually we do NULL for mem_cgroup_disabled(). But that's a
quirk you're inheriting from the existing writeback code.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>