Re: [PATCH v2 4/4] memcg: move memcg private ID refcount to objcg

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

> On Sep 21, 2026, at 14:16, Bingfang Guo via B4 Relay <devnull+bingfangguo.tencent.com@kernel.org> wrote:
> 
> From: Bingfang Guo <bingfangguo@tencent.com>
> 
> The memcg private ID is used by objects that can't afford storing a
> whole pointer and can outlive memcgs to track the memcg (notably swap
> entries). The current design holds a refcount to the css, preventing the
> memcg from being freed.
> 
> This patch unbinds the lifetime of memcgid from the memcg so it can be
> freed.  The idea is to move the refcount of memcgid to one of the
> memcg's objcg. The objcg is stored in the global memcgid xarray instead
> and used for retrieving the online memcg from it.  So swapped out pages
> no longer pin the dying memcg.
> 
> After the change, a memcgid can refer to a non present memcg. To handle
> this situation, when trying to get the original memcg from the id,
> compare the memcgid passed in with that of the memcg, and return NULL to
> indicate its death if they differ. NULL checks are added for
> list_lru_walk_node(), workingset_test_recent() to skip dead memcgs. For
> MGLRU recency test, mem_cgroup_lruvec() will substitute NULL with
> root_mem_cgroup.
> 
> In the earlier patch, an extra xarray lookup was introduced in swap
> uncharging path.  Now that we have the objcg pointer in the function,
> the extra overhead can be removed by using it for putting directly.
> 
> Signed-off-by: Bingfang Guo <bingfangguo@tencent.com>

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

Thanks.