From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-185.mta0.migadu.com (out-185.mta0.migadu.com [91.218.175.185]) (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 7DC643D3306 for ; Sun, 17 May 2026 19:43:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.185 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779047005; cv=none; b=cCfgV2PVyjnMxeXigF9d+q7xZaEll6Kof30KFGrscp0ul1zjgyryn2DHMWTPgKqGjFKPj5fww00pYJ9Dc7RqXpinuo4mC44M9DarXk+yKzLPtyQkLAVeqlZ174Xyw6WwxyH0ExQ8aoRh9B9YaCk6vshAMVo2iqpV7O6ZN2aDqL0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779047005; c=relaxed/simple; bh=KznaCiF5c6LBgkkGNfuVxha2b8yOzd2QqseiC2M76nQ=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=dCFrmvsF0mzBko7McJAm6yEFooRa+GBD38kcakbh5u6eypXLbanw2f2l8XTDAnlnwhrYOZv7/Z3Jcms3LB1TByeGbtGL7eEnzzGRpRQbjmGO2fBDLOibDf6BM0VwR09Alq5cHU8tjnBbV7jjoQ8c/bPhkzGmGqh0csDFQozwbEk= 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=G5XDDwmK; arc=none smtp.client-ip=91.218.175.185 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="G5XDDwmK" 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=1779047001; 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=dVdq1jCkPqSPhxPa9j3o3rCe8JoThsgPaLrsDuG7ZL0=; b=G5XDDwmKqGDrgio9xNyEW3gXjYRXJRVye2jJNq1tfICzAcvzmveTcRuNA7j0+dNrmQXq1N TL9CJp5z/XU/AB1YAilct63QAnncN4XLaNBg8B9pUvG4NH39l7F74XhUjJJKagXU8sBvXV WobS8+JwZE5bH1VoMrALbkYsolkNe6s= From: Shakeel Butt To: Andrew Morton Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot Subject: [PATCH v2] memcg: cache obj_stock by memcg, not by objcg pointer Date: Sun, 17 May 2026 12:43:08 -0700 Message-ID: <20260517194308.952655-1-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 Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type") split a memcg's single obj_cgroup into one per NUMA node, but the per-CPU obj_stock_pcp still keys cached_objcg by pointer. Cross-NUMA workloads now see a drain on every refill and a miss on every consume that targets a sibling per-node objcg of the same memcg, producing the 67.7% stress-ng switch-mq regression reported by LKP. stock->nr_bytes are fungible across per-node objcgs of one memcg. Treat the cache as keyed by memcg in __consume_obj_stock() and __refill_obj_stock() so siblings share the reserve. Compare via READ_ONCE(objcg->memcg) directly: pointer-compare only, no deref, so the rcu_read_lock contract on obj_cgroup_memcg() does not apply. In the same-memcg refill path also fold the incoming objcg's nr_charged_bytes into the stock; otherwise sub-page residue accumulates on whichever sibling was cached at drain time and obj_cgroup_release() silently drops it, leaking up to nr_node_ids * (PAGE_SIZE - 1) bytes per memcg lifecycle from the page_counter. This issue was reported by Sashiko. Update the now-stale invariant comment on __account_obj_stock(). Qi Zheng built a specialized reproducer [1] for the corner case and confirmed the fix. Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type") Link: https://lore.kernel.org/19693be6-7132-446e-b3fc-b7e9f56e5949@linux.dev/ [1] Signed-off-by: Shakeel Butt Debugged-by: Qi Zheng Tested-by: Qi Zheng --- Changes since v1: - Fix the rcu warning (Sashiko). - Fix the page counter possible underflow warning (Sashiko). mm/memcontrol.c | 25 ++++++++++++++++++++++--- 1 file changed, 22 insertions(+), 3 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index d978e18b9b2d..e22ffa3b3319 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3152,7 +3152,12 @@ static void unlock_stock(struct obj_stock_pcp *stock) local_unlock(&obj_stock.lock); } -/* Call after __refill_obj_stock() to ensure stock->cached_objg == objcg */ +/* + * Call after __consume_obj_stock() / __refill_obj_stock(). The stock may be + * cached for a sibling per-node objcg of the same memcg; in that case the + * vmstat batching slot does not match objcg and we fall through to the + * direct path. + */ static void __account_obj_stock(struct obj_cgroup *objcg, struct obj_stock_pcp *stock, int nr, struct pglist_data *pgdat, enum node_stat_item idx) @@ -3210,7 +3215,11 @@ static bool __consume_obj_stock(struct obj_cgroup *objcg, struct obj_stock_pcp *stock, unsigned int nr_bytes) { - if (objcg == READ_ONCE(stock->cached_objcg) && + struct obj_cgroup *cached = READ_ONCE(stock->cached_objcg); + + /* Sibling per-node objcgs share the reserve. */ + if ((cached == objcg || + (cached && READ_ONCE(cached->memcg) == READ_ONCE(objcg->memcg))) && stock->nr_bytes >= nr_bytes) { stock->nr_bytes -= nr_bytes; return true; @@ -3318,6 +3327,7 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes, bool allow_uncharge) { + struct obj_cgroup *cached; unsigned int nr_pages = 0; if (!stock) { @@ -3327,7 +3337,11 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, goto out; } - if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */ + cached = READ_ONCE(stock->cached_objcg); + if (cached == objcg) + goto add_bytes; + /* Direct READ_ONCE due to just pointer comparison. */ + if (!cached || READ_ONCE(cached->memcg) != READ_ONCE(objcg->memcg)) { drain_obj_stock(stock); obj_cgroup_get(objcg); stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes) @@ -3335,7 +3349,12 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, WRITE_ONCE(stock->cached_objcg, objcg); allow_uncharge = true; /* Allow uncharge when objcg changes */ + } else if (atomic_read(&objcg->nr_charged_bytes)) { + /* Fold sibling's stranded ncb into stock; else release leaks it. */ + stock->nr_bytes += atomic_xchg(&objcg->nr_charged_bytes, 0); + allow_uncharge = true; } +add_bytes: stock->nr_bytes += nr_bytes; if (allow_uncharge && (stock->nr_bytes > PAGE_SIZE)) { -- 2.53.0-Meta