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Thu, 12 Nov 2026 07:16:50 -0800 (PST) Received: from localhost ([2605:8600:200:14e3:365a:60ff:fe62:ff29]) by smtp.gmail.com with ESMTPSA id 00721157ae682-8b3933c5a00sm72450787b3.16.2026.11.12.07.16.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Nov 2026 07:16:47 -0800 (PST) Date: Thu, 12 Nov 2026 10:16:46 -0500 From: Johannes Weiner To: Shakeel Butt Cc: Andrew Morton , Dave Chinner , 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-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261112061231.2469026-1-shakeel.butt@linux.dev> X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: F32841C0005 X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Stat-Signature: d5z86bc151pni9oa9kct9wu9ka9rxqpy X-Rspam-User: X-HE-Tag: 1794496611-342605 X-HE-Meta: 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 sRaeWWji 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 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 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 Reviewed-by: Johannes Weiner