From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794495785; b=PF5OaYG1l/LVqk7WTtu15WLH0c74FlSDk7z/jmAx6NB9aSdiQmxmkkx6/8gr1Bax8lYV jhplq8m/+IUF6b56NWN0uK/k632jXZO6nVA+p/sg6+Jn37BFRBW4nh2iUtgNNeBuH9hQQ DlzzaLR7wiUxpJ0yqidaFFsQN0OiB1ygsj6fEvAXGEGGoeLVp1Aw2/Sv7q/tqEENCcDi4 eEf816Q8apYMzMROcVvIGgMjziAHn6nnDEt7o5eFZO1+aJF6MAUhHXjgI707LgupTCUnl thulg0ndhjkttJPRgmltW78thYwgb2R6PvMgI8lc5isCAjEHXvG8/Vn+pF9WYCHXJow== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794495785; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=7K9NJVRHkkfPoN7dlOGzjvSobTAnpjtwzm4XPW/Q2Zk=; b=anwYkTd/qV50frATxPa8fRI9RXDD79J5RHcRJozUlAYwbgQJsSjmgadHSKtPKBFs7y/a U59bS3Kar4TKGj5gMgUirxCVCQWwpwC+JcE08wGXQdWAVuDPZX44OL/1gvDPoePpfsFgI TS12FRZ9P/7p7IGo/2CxzlZgAd1XlAQiurqLk3QUZ7u2xlnx3eKLM3pxd1OtKKAu3rpSb rtQ8QiZ9SpowZ/IGhQ62sTi9aYdYtrlSkHL7hgAiO0aTrciEVnz+WKNnANIbNjxWCsrHl mheqxHXrCjOx9COUY0TpfwrUbw5urAY3xONBIaKOrKEwDdqFnjHpiQw0CmQIV9PE0+A== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=M4gd8OWh; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A1E05606A8A5 for ; Thu, 12 Nov 2026 15:03:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=M4gd8OWh Received: by smtp.kernel.org (Postfix) with ESMTPSA id C30C91F00A06; Thu, 12 Nov 2026 15:03:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794495785; bh=7K9NJVRHkkfPoN7dlOGzjvSobTAnpjtwzm4XPW/Q2Zk=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=M4gd8OWhuhCUm2RCHTG19SJR+Da/kGIIzr30q38JmfIyFEdwGgkXEBSFpTctG7UcT S/ZzgkXA0/a8YDAdRrbn7Sgi8cJMgE9Pxh6GSTU5TfuG7iqjO3LbgtUxZCHmOF4bk8 wSjX7rLlOx30FQ33QK1ajCfqZkLBHPVkDKZ5rJfXth6VsZixQjymwb0JYWVGYX5Opm A7vcUf1saG9qHwg3/HDw6LljAy85DIrPPAIbSe2rDOFHjB/jHahLuOPngEyCDc1SOS JuSHS1oi2U2jpncNJ0nI+HKjurqAXMDMVeZhtweTmgEDBJImEuSC0kkzdEyRscl2My CUhpx4oG6bXNQ== Date: Thu, 12 Nov 2026 15:03:03 +0000 From: Harry Yoo To: Shakeel Butt Cc: Andrew Morton , Dave Chinner , Johannes Weiner , David Hildenbrand , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chris Mason Subject: Re: [PATCH v2] mm: list_lru: fix NULL deref when a folio moves to the root memcg Message-ID: References: <20261112061231.2469026-1-shakeel.butt@linux.dev> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261112061231.2469026-1-shakeel.butt@linux.dev> On Wed, Nov 11, 2026 at 10:12:31PM -0800, Shakeel Butt 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 checking memcg_list_lru_allocated() again in the slow path, > like the fast path does. It returns true for the root, which needs no > lists of its own. > > Fixes: ae64f07a6a40 ("mm: list_lru: introduce folio_memcg_list_lru_alloc()") > Reported-by: Chris Mason > Signed-off-by: Shakeel Butt > --- The fix looks good to me, Reviewed-by: Harry Yoo (Meta) > Changes in v2: > - Check memcg_list_lru_allocated() instead of mem_cgroup_is_root(), like > the fast path does, and update the comment to match (Muchun). > - Link to v1: https://lore.kernel.org/20261112024140.648088-1-shakeel.butt@linux.dev > > mm/list_lru.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/mm/list_lru.c b/mm/list_lru.c > index d1d256822ccc..ca8de3793166 100644 > --- a/mm/list_lru.c > +++ b/mm/list_lru.c > @@ -645,9 +645,16 @@ 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. That memcg may already have its lists, or may be > + * the root, which uses the global lists and has no parent. So > + * check again before allocating. > + */ > memcg = get_mem_cgroup_from_folio(folio); > - res = __memcg_list_lru_alloc(memcg, lru, gfp); > + if (!memcg_list_lru_allocated(memcg, lru)) > + res = __memcg_list_lru_alloc(memcg, lru, gfp); > mem_cgroup_put(memcg); I was wondering how we make sure reparenting doesn't happen after this, but apparently it's fine as __memcg_list_lru_alloc() populates all list_lrus from the memcg up to the hierarchy before the root. -- Cheers, Harry / Hyeonggon