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

[Muchun Song]

On Sun, Jun 19, 2022 at 12:37:32PM -0700, Roman Gushchin wrote:
> 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>
>

Thanks for your review.
 
> 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.
> 

I'll send a new version based on mm-unstable ASAP.

Thanks.

> Thanks!
>