From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lgeamrelo03.lge.com (lgeamrelo03.lge.com [156.147.51.102]) (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 54E75378D8E for ; Mon, 13 Jul 2026 03:11:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=156.147.51.102 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783912317; cv=none; b=i9UTXPIpOYd/sWdTT6qIUUBgIZztWswsQKmfMRWBPyu2s1vrffBnEBd0J3yzVgMrn0G9wxsql23Vg1t3NEgsDK6gGROl9zDkvm67edvCrK+8puUBTt7Ae4eneQajfyb2UrJ2oYNLNKmS0qwQE+QmpK7TcTCrHLnvNF2qS0JxelE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783912317; c=relaxed/simple; bh=XaKmK17oeKnfDQ/ZEiYOB8nUr4mRinwBl1wB5noYncM=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=QvWZLqjAcrHa5G6Vw/G7QDOxKsTxKbtaBTC5AYgqieUi53oT/sz80JGzbt0ikRtS6XjN/zE8/T4MI9KZvLMkB0rkb3M/VijD7HmIJW0bkrHnYv9Kbszye4RUQ5EM7V7cObHhS5d0VxyGNYJH1/81S/VtTEr6NVAq+Mtsz0uTob8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=lge.com; spf=pass smtp.mailfrom=lge.com; arc=none smtp.client-ip=156.147.51.102 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=lge.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=lge.com Received: from unknown (HELO yjaykim-PowerEdge-T330.lge.net) (10.177.112.156) by 156.147.51.102 with ESMTP; 13 Jul 2026 11:56:53 +0900 X-Original-SENDERIP: 10.177.112.156 X-Original-MAILFROM: youngjun.park@lge.com From: Youngjun Park To: akpm@linux-foundation.org Cc: chrisl@kernel.org, youngjun.park@lge.com, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kasong@tencent.com, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, shikemeng@huaweicloud.com, baoquan.he@linux.dev, baohua@kernel.org, yosry@kernel.org, joshua.hahnjy@gmail.com, gunho.lee@lge.com, taejoon.song@lge.com, hyungjun.cho@lge.com, baver.bae@lge.com, her0gyugyu@gmail.com Subject: [PATCH v10 4/6] mm: swap: filter swap allocation by memcg tier mask Date: Mon, 13 Jul 2026 11:56:42 +0900 Message-Id: <20260713025644.170839-5-youngjun.park@lge.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260713025644.170839-1-youngjun.park@lge.com> References: <20260713025644.170839-1-youngjun.park@lge.com> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Apply memcg tier effective mask during swap slot allocation to enforce per-cgroup swap tier restrictions. The folio's effective mask is computed once and passed to the fast, slow and discard paths as a parameter, so all of them act on the same mask even if the memcg's mask changes concurrently. In the fast path, check the percpu cached swap_info's tier_mask against the folio's effective mask. If it does not match, fall through to the slow path. In the slow path, skip swap devices whose tier_mask is not covered by the folio's effective mask. The discard fallback honors the mask too: otherwise it would drain the discard clusters of a device outside the folio's tiers and then loop back to allocate from a tier the memcg is not allowed to use. This works correctly when there is only one non-rotational device in the system and no devices share the same priority. However, there are known limitations: - When non-rotational devices are distributed across multiple tiers, and different memcgs are configured to use those distinct tiers, they may constantly overwrite the shared percpu swap cache. This cache thrashing leads to frequent fast path misses. - Combined with the above issue, if same-priority devices exist among them, a percpu cache miss (overwritten by another memcg) forces the allocator to round-robin to the next device prematurely, even if the current cluster is not fully exhausted. These edge cases do not affect the primary use case of directing swap traffic per cgroup. Further optimization is planned for future work. Signed-off-by: Youngjun Park --- mm/swapfile.c | 24 +++++++++++++++++------- 1 file changed, 17 insertions(+), 7 deletions(-) diff --git a/mm/swapfile.c b/mm/swapfile.c index f3cff586cf30..967399936108 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -1359,7 +1359,7 @@ static bool get_swap_device_info(struct swap_info_struct *si) * Fast path try to get swap entries with specified order from current * CPU's swap entry pool (a cluster). */ -static bool swap_alloc_fast(struct folio *folio) +static bool swap_alloc_fast(struct folio *folio, int mask) { unsigned int order = folio_order(folio); struct swap_cluster_info *ci; @@ -1371,8 +1371,11 @@ static bool swap_alloc_fast(struct folio *folio) * so checking it's liveness by get_swap_device_info is enough. */ si = this_cpu_read(percpu_swap_cluster.si[order]); + if (!si || !swap_tiers_mask_test(si->tier_mask, mask)) + return false; + offset = this_cpu_read(percpu_swap_cluster.offset[order]); - if (!si || !offset || !get_swap_device_info(si)) + if (!offset || !get_swap_device_info(si)) return false; ci = swap_cluster_lock(si, offset); @@ -1389,13 +1392,16 @@ static bool swap_alloc_fast(struct folio *folio) } /* Rotate the device and switch to a new cluster */ -static void swap_alloc_slow(struct folio *folio) +static void swap_alloc_slow(struct folio *folio, int mask) { struct swap_info_struct *si, *next; spin_lock(&swap_avail_lock); start_over: plist_for_each_entry_safe(si, next, &swap_avail_head, avail_list) { + if (!swap_tiers_mask_test(si->tier_mask, mask)) + continue; + /* Rotate the device and switch to a new cluster */ plist_requeue(&si->avail_list, &swap_avail_head); spin_unlock(&swap_avail_lock); @@ -1429,7 +1435,7 @@ static void swap_alloc_slow(struct folio *folio) * Discard pending clusters in a synchronized way when under high pressure. * Return: true if any cluster is discarded. */ -static bool swap_sync_discard(void) +static bool swap_sync_discard(int mask) { bool ret = false; struct swap_info_struct *si, *next; @@ -1437,6 +1443,8 @@ static bool swap_sync_discard(void) spin_lock(&swap_lock); start_over: plist_for_each_entry_safe(si, next, &swap_active_head, list) { + if (!swap_tiers_mask_test(si->tier_mask, mask)) + continue; spin_unlock(&swap_lock); if (get_swap_device_info(si)) { if (si->flags & SWP_PAGE_DISCARD) @@ -1736,6 +1744,7 @@ int folio_alloc_swap(struct folio *folio) { unsigned int order = folio_order(folio); unsigned int size = 1 << order; + int mask; VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); VM_BUG_ON_FOLIO(!folio_test_uptodate(folio), folio); @@ -1759,13 +1768,14 @@ int folio_alloc_swap(struct folio *folio) } again: + mask = folio_tier_effective_mask(folio); local_lock(&percpu_swap_cluster.lock); - if (!swap_alloc_fast(folio)) - swap_alloc_slow(folio); + if (!swap_alloc_fast(folio, mask)) + swap_alloc_slow(folio, mask); local_unlock(&percpu_swap_cluster.lock); if (!order && unlikely(!folio_test_swapcache(folio))) { - if (swap_sync_discard()) + if (swap_sync_discard(mask)) goto again; } -- 2.34.1