From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 CF898302CD5 for ; Mon, 18 May 2026 22:41:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779144066; cv=none; b=O5zbERPL4qJ6kVm297llRMYk8eoyXrhEXbf+VzgpMTBilmfbkSG7vxuUVtzF7/gQnDSSFVRwbHA3RL8kcUQVCKuth7xywSv6Jl+gMM/QFgEp/xDDTWYZ8yZA5qLC5+z1kVybwN2RL8lC9A6VImeKCiLkVHMzw+b8O3KofwCrMhk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779144066; c=relaxed/simple; bh=+GQkobkIiWNMfJQHB8vG5fxx2EdXRH4HYz/I7UJPo2k=; h=Date:To:From:Subject:Message-Id; b=DCGbaSvFuo9sQkEDKmrnByH6wtpqt2zNjRli2mY3g5qpxd4xn5Dbu2MvEzJ0jJG3MYIfZiXXi3gwWAvhUT80Ro3otvKMZCgUJJ9CmPxtr24n4KfcRt3O+zoUSaiUnu173RFWp6EpVF81R+jnrnVWTxDg1MARgpeFMiUM8QYYAgk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=hbCER3Qe; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="hbCER3Qe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 57942C2BCB7; Mon, 18 May 2026 22:41:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1779144066; bh=+GQkobkIiWNMfJQHB8vG5fxx2EdXRH4HYz/I7UJPo2k=; h=Date:To:From:Subject:From; b=hbCER3Qe1+BVxSooNtWa7cB8IplWK1gZRSRItDkK1UF00Gqagysldw3HoTcmFnoGn mEy6FRmb6ZGNoUtRQ1NulJhN4B0hbEXSzgMOsP+gaCCyo3/N8JA0gtC9NyOS3PEDab PF33N20l82YpoH1miXTKhFZnxx7eakTa7MS+DRtM= Date: Mon, 18 May 2026 15:41:05 -0700 To: mm-commits@vger.kernel.org,roman.gushchin@linux.dev,qi.zheng@linux.dev,oliver.sang@intel.com,muchun.song@linux.dev,mhocko@kernel.org,joshua.hahnjy@gmail.com,hannes@cmpxchg.org,alex@ghiti.fr,shakeel.butt@linux.dev,akpm@linux-foundation.org From: Andrew Morton Subject: + memcg-cache-obj_stock-by-memcg-not-by-objcg-pointer.patch added to mm-hotfixes-unstable branch Message-Id: <20260518224106.57942C2BCB7@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: memcg: cache obj_stock by memcg, not by objcg pointer has been added to the -mm mm-hotfixes-unstable branch. Its filename is memcg-cache-obj_stock-by-memcg-not-by-objcg-pointer.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/memcg-cache-obj_stock-by-memcg-not-by-objcg-pointer.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Shakeel Butt Subject: memcg: cache obj_stock by memcg, not by objcg pointer Date: Mon, 18 May 2026 15:28:27 -0700 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. Sharing the reserve without re-caching means bytes funded by one per-node objcg's slow path can be consumed/freed under a different sibling, leaving sub-page residue on whichever sibling was cached at drain time. The pre-existing obj_cgroup_release() path would WARN and silently drop that residue, leaking up to nr_node_ids * (PAGE_SIZE - 1) bytes per memcg lifecycle from the page_counter. Forward the residue into a per-node objcg of the same (post-reparent) memcg at release time instead, so it can be reconciled later via a refill atomic_xchg or another release; the chain terminates at root_mem_cgroup, whose page_counter has no enforced limit. Please note that this is temporary fix and will be reverted when per-node kmem accounting is introduced. Update the stale invariant comment on __account_obj_stock(). Qi Zheng built a specialized reproducer [1] for the corner case and confirmed the fix. Link: https://lore.kernel.org/20260518222827.110696-1-shakeel.butt@linux.dev Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type") Signed-off-by: Shakeel Butt Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com Link: https://lore.kernel.org/19693be6-7132-446e-b3fc-b7e9f56e5949@linux.dev/ [1] Debugged-by: Qi Zheng Cc: Alexandre Ghiti Cc: Johannes Weiner Cc: Joshua Hahn Cc: Michal Hocko Cc: Muchun Song Cc: Roman Gushchin Signed-off-by: Andrew Morton --- mm/memcontrol.c | 69 ++++++++++++++++++++++++++++++++++------------ 1 file changed, 51 insertions(+), 18 deletions(-) --- a/mm/memcontrol.c~memcg-cache-obj_stock-by-memcg-not-by-objcg-pointer +++ a/mm/memcontrol.c @@ -142,14 +142,24 @@ static void obj_cgroup_release(struct pe struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt); unsigned int nr_bytes; unsigned int nr_pages; + unsigned int sub_bytes; unsigned long flags; /* - * At this point all allocated objects are freed, and - * objcg->nr_charged_bytes can't have an arbitrary byte value. - * However, it can be PAGE_SIZE or (x * PAGE_SIZE). + * At this point all allocated objects are freed, but + * objcg->nr_charged_bytes can still hold either + * - (x * PAGE_SIZE) if a small-alloc/drain race left whole pages + * stranded (see the historical sequence below), or + * - any sub-page residue, now that the stock is keyed by memcg and + * sibling per-node objcgs share its reserve: bytes consumed by + * one sibling can spill into another sibling's nr_charged_bytes + * when the stock is drained. * - * The following sequence can lead to it: + * Uncharge the page-aligned portion from this objcg's (post-reparent) + * memcg, and forward any sub-page residue into a per-node objcg of + * the same memcg so it can be reconciled later instead of being lost. + * + * Historical race producing the (x * PAGE_SIZE) case: * 1) CPU0: objcg == stock->cached_objcg * 2) CPU1: we do a small allocation (e.g. 92 bytes), * PAGE_SIZE bytes are charged @@ -160,23 +170,33 @@ static void obj_cgroup_release(struct pe * 92 bytes are added to stock->nr_bytes * 6) CPU0: stock is flushed, * 92 bytes are added to objcg->nr_charged_bytes - * - * In the result, nr_charged_bytes == PAGE_SIZE. - * This page will be uncharged in obj_cgroup_release(). */ nr_bytes = atomic_read(&objcg->nr_charged_bytes); - WARN_ON_ONCE(nr_bytes & (PAGE_SIZE - 1)); nr_pages = nr_bytes >> PAGE_SHIFT; + sub_bytes = nr_bytes & (PAGE_SIZE - 1); - if (nr_pages) { + if (nr_pages || sub_bytes) { struct mem_cgroup *memcg; - memcg = get_mem_cgroup_from_objcg(objcg); - mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages); - memcg1_account_kmem(memcg, -nr_pages); - if (!mem_cgroup_is_root(memcg)) - memcg_uncharge(memcg, nr_pages); - mem_cgroup_put(memcg); + rcu_read_lock(); + memcg = obj_cgroup_memcg(objcg); + + if (nr_pages) { + mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages); + memcg1_account_kmem(memcg, -nr_pages); + if (!mem_cgroup_is_root(memcg)) + memcg_uncharge(memcg, nr_pages); + } + + if (sub_bytes && !mem_cgroup_is_root(memcg)) { + struct obj_cgroup *fwd; + + fwd = rcu_dereference( + memcg->nodeinfo[numa_node_id()]->objcg); + if (fwd) + atomic_add(sub_bytes, &fwd->nr_charged_bytes); + } + rcu_read_unlock(); } spin_lock_irqsave(&objcg_lock, flags); @@ -3152,7 +3172,12 @@ static void unlock_stock(struct obj_stoc 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 fallthrough 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 +3235,11 @@ static bool __consume_obj_stock(struct o 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 +3347,7 @@ static void __refill_obj_stock(struct ob unsigned int nr_bytes, bool allow_uncharge) { + struct obj_cgroup *cached; unsigned int nr_pages = 0; if (!stock) { @@ -3327,7 +3357,10 @@ static void __refill_obj_stock(struct ob goto out; } - if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */ + cached = READ_ONCE(stock->cached_objcg); + /* Direct READ_ONCE due to just pointer comparison. */ + if (cached != objcg && + (!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) _ Patches currently in -mm which might be from shakeel.butt@linux.dev are memcg-cache-obj_stock-by-memcg-not-by-objcg-pointer.patch