Re: [PATCH v3] memcg: keep folio's objcg same as its node

[Muchun Song <muchun.song@linux.dev>]

> On Aug 7, 2026, at 22:24, Shakeel Butt <shakeel.butt@linux.dev> wrote:
> 
> memcg_reparent_objcgs() has an inherent assumption that a folio's objcg
> is the objcg of the folio's node.  Folio migration across nodes breaks
> that assumption: the new folio simply inherits the old folio's objcg
> while living on a different node.
> 
> Once the assumption is broken, the reparenting of the folio's objcg and
> the reparenting of the folio's LRU list are no longer atomic.
> memcg_reparent_objcgs() handles one node per iteration and drops all the
> locks in between, so the objcg gets reparented in the iteration for the
> objcg's node while the LRU list gets spliced in the iteration for the
> folio's node.  Any LRU operation on that folio in between resolves its
> lruvec through the objcg, and thus takes the lru_lock of the wrong
> memcg, not the lru_lock of the list the folio is actually on.
> 
> Fix this by selecting the objcg by folio_nid() at charge time, and by
> re-deriving it for the destination node in mem_cgroup_migrate() and
> mem_cgroup_replace_folio().
> 
> Reported-by: Karl Erik Hofseth <karl.e.hofseth@opoint.com>
> Closes: https://lore.kernel.org/all/anMmd1ADrDVwMO6v@work/
> Fixes: f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios")
> Cc: stable@vger.kernel.org
> Co-developed-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Acked-by: Muchun Song <muchun.song@linux.dev>

Thanks.