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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> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: hspsmetin8uazhcwsk7quky5czucohxt X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: CB8EF1A0002 X-Rspam-User: X-HE-Tag: 1779047004-21285 X-HE-Meta: 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 lMXL9UYY pnZoH1pRBCj0TDcItXUHT6mz5IIlfozBscP4cO6QrTZTx27IvrV0Emvqat+VI9T+gWsLqijZwVTKho4LL126zh6nXZ9RoQB77OTE7tZI7aNh3BSkHIJD77TMmv0Cc6kgzrYbH1E9vW+EBoIEvd7nWZf0AtBTXuT4IGhhc3uTzQPcq62osHYbunAXhGnA06hL2yE1c/G8LZkAWfhIh8giTRdFRX7Ht6VT5cfmO+SIBNRHQ5/yNd1ZhGZd0zn53fDq4ydSdr/A/eOyPzBCkAglO9ZvTMcNxxQl0bj1m Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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