Re: [PATCH] mm: list_lru: fix NULL deref when a folio moves to the root memcg

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

> On Nov 12, 2026, at 10:41, Shakeel Butt <shakeel.butt@linux.dev> wrote:
> 
> folio_memcg_list_lru_alloc() makes sure that a folio's memcg has its
> own lists in a list_lru, such as the THP deferred split queue. It looks
> up the folio's memcg twice: a fast path under RCU checks if the lists
> exist, and a slow path takes a reference and allocates them. The fast
> path returns early for the root memcg, which uses the global lists.
> The slow path does not check for the root.
> 
> The memcg can change between the two lookups: when a cgroup is
> removed, its folios are moved to the parent memcg. If the folio is
> moved up to the root in between, the slow path gets the root, and
> __memcg_list_lru_alloc() dereferences the root's parent, which is NULL.
> 
> Users see a NULL pointer dereference oops while a THP is allocated,
> such as in a page fault, in khugepaged or at swap-in. tmpfs huge pages
> are affected too, and for them the oops can also happen in reclaim,
> for example in kswapd. The task that hits the oops is killed, or the
> machine panics if panic_on_oops is set.
> 
> The race is narrow: the folio has to be moved up to the root right
> between the two lookups. For example, its cgroup is directly below the
> root and is removed, or has its memory controller disabled, at that
> moment.
> 
> Fix it by skipping the root memcg in the slow path too. The root needs
> no lists of its own.
> 
> Fixes: ae64f07a6a40 ("mm: list_lru: introduce folio_memcg_list_lru_alloc()")
> Reported-by: Chris Mason <mason@kernel.org>
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> ---
> mm/list_lru.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index d1d256822ccc..566691ca3dd8 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -645,9 +645,15 @@ int folio_memcg_list_lru_alloc(struct folio *folio, struct list_lru *lru,
> 	if (likely(res))
> 		return 0;
> 
> -	/* Allocation may block, pin the memcg */
> +	/*
> +	 * Allocation may block, so pin the memcg. If the folio's cgroup
> +	 * was removed after the check above, the folio has moved up to a
> +	 * parent memcg, which can be the root. The root uses the global
> +	 * lists and has no parent, so skip it.
> +	 */
> 	memcg = get_mem_cgroup_from_folio(folio);
> -	res = __memcg_list_lru_alloc(memcg, lru, gfp);
> +	if (!mem_cgroup_is_root(memcg))

How about using memcg_list_lru_allocated() to check this, it's consistent with
the above check?

Thanks,
Muchun

> +		res = __memcg_list_lru_alloc(memcg, lru, gfp);
> 	mem_cgroup_put(memcg);
> 	return res;
> }
> -- 
> 2.53.0-Meta
>