From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta1.migadu.com (out-182.mta1.migadu.com [95.215.58.182]) (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 4BC13441054 for ; Wed, 3 Jun 2026 11:53:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780487618; cv=none; b=mRL70DmaWd1FzHLqiYbMO8OKAD/4fT/LzzhQGlqLs6JPjaSqLNdQW5pSX88JAK/V9tqO+eo3Wyrvoi4vYMv4ROn1QWI/VE6Dr53F5gatqyPgk8LC7ak5H5wY42j94EAgzYMCTXxekHhDZVrXdPbLQAljmWpOLkBKUZzOoOHmBcg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780487618; c=relaxed/simple; bh=xV5X40UJB+xxHtXRfMMr2kdqCKKSoanWJV4O5fQjubE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iNMUlPjHy5h5qM67jbq43pOzR1DaJN0ajhNt2zbuSLVh4bmORktW/4tG7a0X/U0i+/OAGH4FwjQj1GrR9TMCqAFAC8D9bdFgshJBpItAmfuPJuLTLrqaZ2CF7vTWuWUCusWZYaqMGKFixz6aKMYDjWqjC/UnCjDw1/KWc9/B0XU= 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=X/BAccuz; arc=none smtp.client-ip=95.215.58.182 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="X/BAccuz" Message-ID: <4b2ebbf7-8c5e-45ca-a17c-111f68e2324c@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780487613; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=oO32gU/6GqPqmsbPD/EqqtNbEEfCeRvWof7NPW20STY=; b=X/BAccuz5VRKWoWp8Tw5a2hyusMTKMdhoDQBENIqijF2kktRaXNzuLf0junIel8CkvdmcH 7CkezdTUzOvIYcG1228DGzW2sck4oSGXaI2Nlo29boIh8aW+zNXC1v/kuuOV14NgNLfCos zI20XgSI/HCz3BejCUKEmsHCU+3KC+A= Date: Wed, 3 Jun 2026 19:53:17 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v5 0/9] mm: switch THP shrinker to list_lru To: Johannes Weiner Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, shakeel.butt@linux.dev, mhocko@kernel.org, david@fromorbit.com, roman.gushchin@linux.dev, muchun.song@linux.dev, qi.zheng@linux.dev, yosry.ahmed@linux.dev, ziy@nvidia.com, liam@infradead.org, usama.arif@linux.dev, kas@kernel.org, vbabka@kernel.org, ryncsn@gmail.com, zaslonko@linux.ibm.com, gor@linux.ibm.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, dev.jain@arm.com, npache@redhat.com, ryan.roberts@arm.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260603044426.54863-1-lance.yang@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2026/6/3 19:41, Johannes Weiner wrote: > On Wed, Jun 03, 2026 at 12:44:26PM +0800, Lance Yang wrote: >> >> On Tue, Jun 02, 2026 at 05:46:02PM -0400, Johannes Weiner wrote: >>> On Mon, Jun 01, 2026 at 04:36:52PM +0800, Lance Yang wrote: >>>> As the changelog above says, the old queue is per-memcg only, rather >>>> than per-memcg-per-node. So reclaim on one node can still walk the whole >>>> memcg queue and split underused THPs from other nodes in the same memcg. >>>> >>>> But I think the new one can lose reclaim in the cgroup.memory=nokmem >>>> case ... >>>> >>>> With nokmem, the deferred shrinker can still run from memcg reclaim, >>>> because it is SHRINKER_NONSLAB. But the list_lru is no longer per-memcg: >>>> >>>> __list_lru_init() clears memcg_aware, >>>> >>>> if (mem_cgroup_kmem_disabled()) >>>> memcg_aware = false; >>>> >>>> so list_lru_from_memcg_idx() falls back to the shared node list: >>>> >>>> static inline struct list_lru_one * >>>> list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx) >>>> { >>>> if (list_lru_memcg_aware(lru) && idx >= 0) { >>>> [...] >>>> } >>>> return &lru->node[nid].lru; >>>> } >>>> >>>> That makes the shrinker bit unreliable. __list_lru_add() still sets the >>>> bit on the memcg passed in, but only when the list goes from empty to >>>> non-empty: >>>> >>>> bool __list_lru_add(struct list_lru *lru, struct list_lru_one *l, >>>> struct list_head *item, int nid, >>>> struct mem_cgroup *memcg) >>>> { >>>> if (list_empty(item)) { >>>> [...] >>>> if (!l->nr_items++) >>>> set_shrinker_bit(memcg, nid, lru_shrinker_id(lru)); >>>> [...] >>>> return true; >>>> } >>>> return false; >>>> } >>>> >>>> If memcg A adds the first folio, A gets the bit. If memcg B later adds a >>>> folio to the same shared list, B does not get a bit, because the list >>>> was already non-empty. >>>> >>>> So in the A-first/B-later case, reclaim from B may not call the deferred >>>> shrinker at all. The shared list is scanned from memcg reclaim only if >>>> reclaim runs from the memcg that has the bit, such as A here, or from >>>> global reclaim :) >>>> >>>> Anyway, only after the shared list is emptied does the next memcg to add >>>> a folio get to be the one with the bit, IIUC :) >>> >>> Sorry for the delay, this took me a bit to think about. The shrinker >>> code is a mess. >>> >>> I read it the same way you do. And this is true for all list_lru users >>> when nokmem is set: we just set random nonsense shrinker bits. >>> >>> HOWEVER, the generic shrinker code fixes that up by IGNORING random >>> shrinker bits like this when !memcg_kmem_online(). And shrinking >>> correctly happens only against the shared root queue when the reclaim >>> iterator walks root_mem_cgroup. >>> >>> HOWEVER, the THP shrinker explicitly sets SHRINKER_NONSLAB, which in >>> turn overrides the previous override. So yes there is a weirdness: we >>> get the root cgroup invocation against the shared queue, and then one >>> more time triggered by that random memcg bit. >>> >>> The most direct fix is to just drop SHRINKER_NONSLAB. It declares >>> independence from kmem, which is no longer true. >>> >>> Cleaning up the shrinker code is left for another day. >> >> Thanks for working on this! >> >> Wondering if this fix trades one problem for another, though ... >> >> Before this series, the deferred split shrinker had a real per-memcg >> queue. Even with cgroup.memory=nokmem, memcg reclaim could still scan >> that memcg's own deferred_split_queue: >> >> memcg reclaim -> deferred split shrinker -> sc->memcg->deferred_split_queue >> >> With the fix, nokmem + w/o SHRINKER_NONSLAB falls back to a >> non-memcg-aware shrinker: >> >> memcg reclaim -> skip deferred split shrinker >> >> root/global reclaim -> deferred split shrinker -> shared list_lru >> >> Is that expected? There woud be no memcg-driven deferred split reclaim >> under nokmem, IIUC ... > > Yes, this is all correct. list_lru is still inherently tied to the > kmem component of memcg (memcg_kmem_id()). > > So without kmem, no isolation. But without kmem, no isolation *for a > lot of stuff*. It's a legacy knob when slab accounting was new and > expensive. But so many things depend on it now, disabling it just > punches a nassive hole into memcg functionality and isolation > coverage. It's not a sanctioned production use flag. > > This change is negligible from a memcg semantics POV. Thanks for clarifying! No strong objection from me. Just wanted to call out the nokmem behavior change and hear what folks think :D >> Not sure what the right fix is, as I am not a memcg expert ...