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 8428D36AB74 for ; Wed, 18 Mar 2026 20:25:23 +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=1773865523; cv=none; b=tDcewSVOxHhpWnodTx2/HnJ4NNKzwXeEe/tqRqmuWy2sPJvvYfO9WTSroV0ShSd8ObJ+Wanki9x3nIdsCUxzq4sq8OaQGzfr0f6V9UuOutBMfMsPrFNdXJvKSJ7l2XzKFUCkaIxE5cQ0OsZjgev2SKQZa5GWATYUvz/Y+GzL4D8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773865523; c=relaxed/simple; bh=mdvbQ1tn7ST4LxwKLY9jxhU4Fn365EEeLx0lD1neL3I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=q62pXArQM+mG4LrE0EMpUUsoiKcXxNKJvk+Ph7eyIl3My464V9s1diiTiFjmbzZSVyMNoVLuhnrzaiYkDUHfAB11XV7KI/Ji2QV+cdcWSvrJ00BYatqxCNdTDsdfWcgNC25NhxKjOOD3T4cPI99XTO/du2L7Dseh1vN2Ag0jxgQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Axa2bfrR; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Axa2bfrR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D8E86C19421; Wed, 18 Mar 2026 20:25:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773865523; bh=mdvbQ1tn7ST4LxwKLY9jxhU4Fn365EEeLx0lD1neL3I=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Axa2bfrR+vFXIn2OLHz5yzvI8zsrqvlpYIiRcZlfngiCW0CtrIuol0gm+Ro2gmT9J DVO68Xz4neacd/yVCLpdAbNtDgcivQVKdbFYpUHSQl3eQVEKFmsLDspTnpwHV1O5ed 3Jr/JyxK84pWs+MGBEcsRLxX2+cUWRImJwlnMvReLphGJ9IZjGn32CxvK8bgAjpGpn gzgxm7zA1H7gJqKv5o7NcFHPo8FzdTtfGRtMhq96YDP6Eazd560lR38exv1Nu9InLq SSrSGOIKV7Ori6NRExhZ3KfGIOHpOoM2tqJ5NOXOomU64HIqI2N+rfqD2IBntmXeN3 zTWX0+Uare4yQ== Message-ID: <61d86249-cd89-4e99-99d8-ab7c72e95f34@kernel.org> Date: Wed, 18 Mar 2026 21:25:17 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 7/7] mm: switch deferred split shrinker to list_lru To: Johannes Weiner , Andrew Morton Cc: Shakeel Butt , Yosry Ahmed , Zi Yan , "Liam R. Howlett" , Usama Arif , Kiryl Shutsemau , Dave Chinner , Roman Gushchin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260312205321.638053-1-hannes@cmpxchg.org> <20260312205321.638053-8-hannes@cmpxchg.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260312205321.638053-8-hannes@cmpxchg.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/12/26 21:51, Johannes Weiner wrote: > The deferred split queue handles cgroups in a suboptimal fashion. The > queue is per-NUMA node or per-cgroup, not the intersection. That means > on a cgrouped system, a node-restricted allocation entering reclaim > can end up splitting large pages on other nodes: > > alloc/unmap > deferred_split_folio() > list_add_tail(memcg->split_queue) > set_shrinker_bit(memcg, node, deferred_shrinker_id) > > for_each_zone_zonelist_nodemask(restricted_nodes) > mem_cgroup_iter() > shrink_slab(node, memcg) > shrink_slab_memcg(node, memcg) > if test_shrinker_bit(memcg, node, deferred_shrinker_id) > deferred_split_scan() > walks memcg->split_queue > > The shrinker bit adds an imperfect guard rail. As soon as the cgroup > has a single large page on the node of interest, all large pages owned > by that memcg, including those on other nodes, will be split. > > list_lru properly sets up per-node, per-cgroup lists. As a bonus, it > streamlines a lot of the list operations and reclaim walks. It's used > widely by other major shrinkers already. Convert the deferred split > queue as well. > > The list_lru per-memcg heads are instantiated on demand when the first > object of interest is allocated for a cgroup, by calling > memcg_list_lru_alloc_folio(). Add calls to where splittable pages are > created: anon faults, swapin faults, khugepaged collapse. > > These calls create all possible node heads for the cgroup at once, so > the migration code (between nodes) doesn't need any special care. [...] > - > static inline bool is_transparent_hugepage(const struct folio *folio) > { > if (!folio_test_large(folio)) > @@ -1293,6 +1189,14 @@ static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma, > count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE); > return NULL; > } > + > + if (memcg_list_lru_alloc_folio(folio, &deferred_split_lru, gfp)) { > + folio_put(folio); > + count_vm_event(THP_FAULT_FALLBACK); > + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK); > + return NULL; > + } So, in all anon alloc paths, we essentialy have 1) vma_alloc_folio / __folio_alloc (khugepaged being odd) 2) mem_cgroup_charge / mem_cgroup_swapin_charge_folio 3) memcg_list_lru_alloc_folio I wonder if we could do better in most cases and have something like a vma_alloc_anon_folio() That wraps the vma_alloc_folio() + memcg_list_lru_alloc_folio(), but still leaves the charging to the caller? The would at least combine 1) and 3) in a single API. (except for the odd cases without a VMA). I guess we would want to skip the memcg_list_lru_alloc_folio() for order-0 folios, correct? > + > folio_throttle_swaprate(folio, gfp); > > /* > @@ -3802,33 +3706,28 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n > struct folio *new_folio, *next; > int old_order = folio_order(folio); > int ret = 0; > - struct deferred_split *ds_queue; > + struct list_lru_one *l; > > VM_WARN_ON_ONCE(!mapping && end); > /* Prevent deferred_split_scan() touching ->_refcount */ > - ds_queue = folio_split_queue_lock(folio); > + rcu_read_lock(); The RCU lock is for the folio_memcg(), right? I recall I raised in the past that some get/put-like logic (that wraps the rcu_read_lock() + folio_memcg()) might make this a lot easier to get. memcg = folio_memcg_lookup(folio) ... do stuff folio_memcg_putback(folio, memcg); Or sth like that. Alternativey, you could have some helpers that do the list_lru_lock+unlock etc. folio_memcg_list_lru_lock() ... folio_memcg_list_ru_unlock(l); Just some thoughts as inspiration :) > + l = list_lru_lock(&deferred_split_lru, folio_nid(folio), folio_memcg(folio)); > if (folio_ref_freeze(folio, folio_cache_ref_count(folio) + 1)) { > struct swap_cluster_info *ci = NULL; > struct lruvec *lruvec; > > if (old_order > 1) { > - if (!list_empty(&folio->_deferred_list)) { > - ds_queue->split_queue_len--; > - /* > - * Reinitialize page_deferred_list after removing the > - * page from the split_queue, otherwise a subsequent > - * split will see list corruption when checking the > - * page_deferred_list. > - */ > - list_del_init(&folio->_deferred_list); > - } > + __list_lru_del(&deferred_split_lru, l, > + &folio->_deferred_list, folio_nid(folio)); > if (folio_test_partially_mapped(folio)) { > folio_clear_partially_mapped(folio); > mod_mthp_stat(old_order, > MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1); > } > } > - split_queue_unlock(ds_queue); > + list_lru_unlock(l); > + rcu_read_unlock(); > + > if (mapping) { [...] Most changes here look mostly mechanically, quite nice. I'll probably have to go over some bits once again with a fresh mind :) -- Cheers, David