From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta0.migadu.com (out-172.mta0.migadu.com [91.218.175.172]) (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 104893321B7 for ; Mon, 1 Dec 2025 15:41:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764603705; cv=none; b=JdHXuSBwCCN5o2rtsbtbkFr+6YV2HhitqhFUdBZ+Lc/fGCdx4t3GtsH6lcF6TJ1s02waxMd3V848jcItTsHYsT6EG/1S4Za1qxnJMgYwbw1Xm5l9J158JvPagw3HBno5dRzZxnnRwtQF+U0Wc90rbjQWpS6Ykpfi8g/ixmc9CeM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764603705; c=relaxed/simple; bh=xUCZRq2Rv9QQoZ9a9Ydts73QvuVRqGYF2LyekzomqEM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nsmZeNxKWo6nWbgMVD/SsbdMoH/QVqizashyWoVPG7ix/8m5nFpNSLZthIdDerV/YPcy8hR7e0sfypospa4HoDMTXhrPoFjCOCV3I3xPSU2gp5YzCrNKj866UTF6bteBOkgaxMF6/iuXoxVdQv0t6wKaFwxC3W5WTXgtaTJqsJQ= 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=ZzA+zGcb; arc=none smtp.client-ip=91.218.175.172 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="ZzA+zGcb" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1764603699; 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=uT9+iCk1C9W8oKaATIUH1G8jZwnmHrr4Jk2aweSPxhw=; b=ZzA+zGcbfVaF0IbxZAcyuNjv0u24QwW+Exx11JVnVgnKJMNQ876Zu3WzrgtNGfGB9g2W3l pZR6eaHFu4H43WJN2dh8RPmTqQqQ/THi68oYhI6t26RqdFVxCh7Zuzpo8Cx2XdFvQCovCb bdvKKrcGaIP6J4hvwcUw4QyzEr+TKNc= Date: Mon, 1 Dec 2025 23:40:38 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v1 23/26] mm: vmscan: prepare for reparenting MGLRU folios To: Harry Yoo Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, david@redhat.com, lorenzo.stoakes@oracle.com, ziy@nvidia.com, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Qi Zheng References: X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 11/26/25 9:48 PM, Harry Yoo wrote: > On Tue, Oct 28, 2025 at 09:58:36PM +0800, Qi Zheng wrote: >> From: Qi Zheng >> >> Similar to traditional LRU folios, in order to solve the dying memcg >> problem, we also need to reparenting MGLRU folios to the parent memcg when >> memcg offline. >> >> However, there are the following challenges: >> >> 1. Each lruvec has between MIN_NR_GENS and MAX_NR_GENS generations, the >> number of generations of the parent and child memcg may be different, >> so we cannot simply transfer MGLRU folios in the child memcg to the >> parent memcg as we did for traditional LRU folios. >> 2. The generation information is stored in folio->flags, but we cannot >> traverse these folios while holding the lru lock, otherwise it may >> cause softlockup. >> 3. In walk_update_folio(), the gen of folio and corresponding lru size >> may be updated, but the folio is not immediately moved to the >> corresponding lru list. Therefore, there may be folios of different >> generations on an LRU list. >> 4. In lru_gen_del_folio(), the generation to which the folio belongs is >> found based on the generation information in folio->flags, and the >> corresponding LRU size will be updated. Therefore, we need to update >> the lru size correctly during reparenting, otherwise the lru size may >> be updated incorrectly in lru_gen_del_folio(). >> >> Finally, this patch chose a compromise method, which is to splice the lru >> list in the child memcg to the lru list of the same generation in the >> parent memcg during reparenting. And in order to ensure that the parent >> memcg has the same generation, we need to increase the generations in the >> parent memcg to the MAX_NR_GENS before reparenting. >> >> Of course, the same generation has different meanings in the parent and >> child memcg, this will cause confusion in the hot and cold information of >> folios. But other than that, this method is simple enough, the lru size >> is correct, and there is no need to consider some concurrency issues (such >> as lru_gen_del_folio()). >> >> To prepare for the above work, this commit implements the specific >> functions, which will be used during reparenting. >> >> Suggested-by: Harry Yoo >> Suggested-by: Imran Khan >> Signed-off-by: Qi Zheng >> --- >> include/linux/mmzone.h | 16 ++++++++ >> mm/vmscan.c | 86 ++++++++++++++++++++++++++++++++++++++++++ >> 2 files changed, 102 insertions(+) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index 7aa8e1472d10d..3ee7fb96b8aeb 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -4468,6 +4468,92 @@ void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid) >> lru_gen_rotate_memcg(lruvec, MEMCG_LRU_HEAD); >> } >> >> +bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg) >> +{ >> + int nid; >> + >> + for_each_node(nid) { >> + struct lruvec *lruvec = get_lruvec(memcg, nid); >> + int type; >> + >> + for (type = 0; type < ANON_AND_FILE; type++) { >> + if (get_nr_gens(lruvec, type) != MAX_NR_GENS) >> + return false; >> + } >> + } >> + >> + return true; >> +} >> + >> +/* >> + * We need to ensure that the folios of child memcg can be reparented to the >> + * same gen of the parent memcg, so the gens of the parent memcg needed be >> + * incremented to the MAX_NR_GENS before reparenting. >> + */ >> +void max_lru_gen_memcg(struct mem_cgroup *memcg) >> +{ >> + int nid; >> + >> + for_each_node(nid) { >> + struct lruvec *lruvec = get_lruvec(memcg, nid); >> + int type; >> + > > I was testing this series and observed two warnings... > >> + for (type = 0; type < ANON_AND_FILE; type++) { >> + while (get_nr_gens(lruvec, type) < MAX_NR_GENS) { >> + DEFINE_MAX_SEQ(lruvec); >> + >> + inc_max_seq(lruvec, max_seq, mem_cgroup_swappiness(memcg)); >> + cond_resched(); > > Warning 1) Here we increment max_seq but we skip updating mm_state->seq. > (try_to_inc_max_seq() iterates the mm list and update mm_state->seq after > an iteration, but since we directly call inc_max_seq(), we don't update it) > > When mm_state->seq is more than one generation behind walk->seq, a warning is > triggered in iterate_mm_list(): > > VM_WARN_ON_ONCE(mm_state->seq + 1 < walk->max_seq); The mm_state->seq is just to record the completion of a full traversal of mm_list. If we simply delete this warning, it may cause this judgment in iterate_mm_list to become invalid: if (walk->seq <= mm_state->seq) goto done; So it seems we can manually increase mm_state->seq during reparenting to avoid this warning. However, we cannot directly call iterate_mm_list_nowalk() because we do not want to reset mm_state->head and mm_state->tail to NULL. Otherwise, we wouldn't be able to continue iterating over the mm_list. > > Warning 2) In try_to_inc_max_seq(), the last walker of mm list > is supposed to succeed to increment max_seq by calling inc_max_seq(): > > if (success) { > success = inc_max_seq(lruvec, seq, swappiness); > WARN_ON_ONCE(!success); > } > > But with this patch it may observe the max_seq is already advanced due to > reparenting and thus inc_max_seq() returns false, triggering the warning. After we correct the warning above, we will satisfy this condition in iterate_mm_list(): if (walk->seq <= mm_state->seq) goto done; thus returning false, and avoiding triggering this warning. > > I'm learning MGLRU internals to see whether we can simply remove the warnings > or if we need to do something to advance max_seq without actually iterating > over the mm list. So IIUC, we can simple increase mm_state->seq during reparenting to fix these warnings. >