From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta1.migadu.com (out-180.mta1.migadu.com [95.215.58.180]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 789FE35F602 for ; Mon, 1 Jun 2026 06:34:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780295683; cv=none; b=l3AU0xtWARkIQ+FtFdZ+PgH1T/B/x/RdA9xg2GMWqfOTsMJF5qpH6p70/3S3BuDznmci9fSQAwTaQxUlUQ1NWLUBiK88jltrVFYE4hRA2MZCzw8fR540pt5LoS4lpJwtcvn9RrI/XaKxuFxdbd9mPnylZPYj5IiJ5m4E13T2L2I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780295683; c=relaxed/simple; bh=qqBWByypUKq2FsizaIPyo4COr5T8Ke5xWvws1jGf5p8=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=nRe49ZzVlG+ydhR9gE92OrRu3VU3jsYmQt7ju9u/mC0vzAeTsidg3XI6Lgk1QRKgU5Fn3BT0KCZHK3BabAjaz2z0e1HyV0uUBe1ySVJirK30alW+c0TmAK+6Rjfj/E1E5JOIh0HUzQSv8oVnNtvB6CmIm0LKWavhrL3Wk8lJbAU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=Ngy8hvkP; arc=none smtp.client-ip=95.215.58.180 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="Ngy8hvkP" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780295679; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=QiRzdMswPgawDFLq5HRk8E4c2GK+uK4ZKIh/iRxMht8=; b=Ngy8hvkPHXVBJz6JxI/RWTNHm8G9rzOSN+P+SvwhC3BCDsRo0S8N97K8OJN2VBDNdnG52P /uv1anMx9+ccsvLgSQT869Dzi8ZbFTKpTdkaSyZlnHISVsVlstrE44d3lKJse6MCqw6d0l 9VKmneou8Ozl4GvRBqHN3Pug+JGxW6M= From: Shakeel Butt To: Andrew Morton Cc: Johannes Weiner , Dave Chinner , Roman Gushchin , Muchun Song , Qi Zheng , Kairui Song , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Chris Mason Subject: [PATCH] mm/list_lru: drain before clearing xarray entry on reparent Date: Sun, 31 May 2026 23:34:08 -0700 Message-ID: <20260601063408.2879011-1-shakeel.butt@linux.dev> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT memcg_reparent_list_lrus() clears the dying memcg's xarray entry with xas_store(&xas, NULL) before reparenting its per-node lists into the parent. This opens a window where a concurrent list_lru_del() arriving for the dying memcg sees xa_load() == NULL, walks to the parent in lock_list_lru_of_memcg(), takes the parent's per-node lock, and calls list_del_init() on an item still physically linked on the dying memcg's list. If another in-flight thread holds the dying memcg's per-node lock at the same moment (another list_lru_del, or a list_lru_walk_one running an isolate callback), both threads modify ->next/->prev pointers on the same physical list under different locks. Adjacent items can corrupt each other's links. Fix it by reversing the order: reparent each per-node list and mark the child's list lru dead and then clear the xarray entry. Any concurrent list_lru op that finds the still-set xarray entry either takes the dying memcg's per-node lock (synchronizing with the drain) or sees LONG_MIN and walks to the parent, where the items now live. Fixes: fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope") Signed-off-by: Shakeel Butt Reported-by: Chris Mason --- mm/list_lru.c | 20 +++++++++----------- 1 file changed, 9 insertions(+), 11 deletions(-) diff --git a/mm/list_lru.c b/mm/list_lru.c index dd29bcf8eb5f..ae55a52307db 100644 --- a/mm/list_lru.c +++ b/mm/list_lru.c @@ -473,26 +473,24 @@ void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *paren mutex_lock(&list_lrus_mutex); list_for_each_entry(lru, &memcg_list_lrus, list) { struct list_lru_memcg *mlru; - XA_STATE(xas, &lru->xa, memcg->kmemcg_id); - /* - * Lock the Xarray to ensure no on going list_lru_memcg - * allocation and further allocation will see css_is_dying(). - */ - xas_lock_irq(&xas); - mlru = xas_store(&xas, NULL); - xas_unlock_irq(&xas); + mlru = xa_load(&lru->xa, memcg->kmemcg_id); if (!mlru) continue; /* - * With Xarray value set to NULL, holding the lru lock below - * prevents list_lru_{add,del,isolate} from touching the lru, - * safe to reparent. + * Reparent each per-node list and mark the child dead + * (LONG_MIN) before clearing xarray entry otherwisw a + * concurrent list_lru_del() may corrupt the list if it arrives + * after xarray clear but before reparenting as + * lock_list_lru_of_memcg will acquire parent's lock while the + * item is still on child's list. */ for_each_node(i) memcg_reparent_list_lru_one(lru, i, &mlru->node[i], parent); + xa_erase(&lru->xa, memcg->kmemcg_id); + /* * Here all list_lrus corresponding to the cgroup are guaranteed * to remain empty, we can safely free this lru, any further -- 2.52.0