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Thu, 12 Nov 2026 05:44:22 +0000 X-Mizu-Trace-ID: 5c3232aa448fa3e9 X-Migadu-Flow: FLOW_OUT Date: Wed, 11 Nov 2026 21:44:16 -0800 From: Shakeel Butt To: Muchun Song Cc: Andrew Morton , Dave Chinner , Johannes Weiner , David Hildenbrand , Michal Hocko , Roman Gushchin , Qi Zheng , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chris Mason Subject: Re: [PATCH] mm: list_lru: fix NULL deref when a folio moves to the root memcg Message-ID: References: <20261112024140.648088-1-shakeel.butt@linux.dev> <26CC0CD2-97FE-4842-AE51-605EC77AC91D@linux.dev> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <26CC0CD2-97FE-4842-AE51-605EC77AC91D@linux.dev> X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: EDFA140003 X-Stat-Signature: 4ktk55g7pzrs61s1j935i6cu7kk7aps1 X-Rspam-User: X-HE-Tag: 1794462264-179214 X-HE-Meta: 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 LUwz57Uu V2agpg610b0LXeuGe0Rbe9gFHhVBBNhim2ZNlu4kIJ/9z0fJ5J4sWpjAE5B5yowlEsv7Cx73ZnN22SDmbXUOmqrn7JKPfVwNYmI40gGAZGkN9pNE85up5/8AiZcvVcELu1K6j/vpz/MLDTc/JoNZv7v8GX17AIb5jfJyCbOGVR63YoX6kgnlfREYp/BBZpjos7+o0JDlitVvcAveYh7MyohqN1DqwpQOw6zw+JuPJy6Ho2dFGjICu5s1rF9Mrm6GdnNMmnS/5tCwkhFnwXNbY/kb4j0B5cfSeRy4Ky8uQD1a+b+FyKszeKsuYujuh3V8+xE6lt+CsiPoP+6pSUNm58spTP4KqQd1HrAZ/jX6DnqVMewuxRuGVar15gA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Thu, Nov 12, 2026 at 10:49:44AM +0800, Muchun Song wrote: > > > > On Nov 12, 2026, at 10:41, 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 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 > > Signed-off-by: Shakeel Butt > > > > --- > > 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? Yes, that makes sense. Will send out v2.