Re: [PATCH v5 07/11] mm: memcontrol: make all the callers of {folio,page}_memcg() safe

[Roman Gushchin]

On Mon, May 30, 2022 at 03:49:15PM +0800, Muchun Song wrote:
> When we use objcg APIs to charge the LRU pages, the page will not hold
> a reference to the memcg associated with the page. So the caller of the
> {folio,page}_memcg() should hold an rcu read lock or obtain a reference
> to the memcg associated with the page to protect memcg from being
> released. So introduce get_mem_cgroup_from_{page,folio}() to obtain a
> reference to the memory cgroup associated with the page.
> 
> In this patch, make all the callers hold an rcu read lock or obtain a
> reference to the memcg to protect memcg from being released when the LRU
> pages reparented.
> 
> We do not need to adjust the callers of {folio,page}_memcg() during
> the whole process of mem_cgroup_move_task(). Because the cgroup migration
> and memory cgroup offlining are serialized by @cgroup_mutex. In this
> routine, the LRU pages cannot be reparented to its parent memory cgroup.
> So {folio,page}_memcg() is stable and cannot be released.
> 
> This is a preparation for reparenting the LRU pages.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>

Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>

The locking seems to be correct. I'm slightly worried about a potential
perf degradation, especially on dying cgroups, where css_get() is relatively
expensive. I hope getting it into mm-unstable will help to determine
whether it's actually a problem.

Thanks!