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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=DvxGAV+O; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B2B22606A31B for ; Tue, 20 Oct 2026 07:55:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=DvxGAV+O Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5174B1F000C4; Tue, 20 Oct 2026 07:55:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792482959; bh=mG3kNywBpCjlsN90sZBCwLqBblspQOk2h7zQBNKp7wI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=DvxGAV+OaraT9ljQafttLtkjrCxCFCziGjRjdD6HwDSpmr4+0+MkXiqlz3dN3LkvR MfM/McOKEjS6LdRZ6VGYT8CWPrJhfnxWW1JkPviBcxyJrcCOm6OYTyoDRKnLxDzXlJ gygE0Udc9/JKsQXMuoyTGMOiyu0lM2/HJ4EK04aOfsQ60EhPsIji7jnMWfon937KHA Ry9q64jMwMhte7oWL6nK2TbfURYsSEwUAOTeWoV8lik4nsuFWIv4b7hJNTI2x4/F8O 7YwJlW32gApnjMaxcZR64YRoxdu+mtO7KqcMSGUdLoFSnKOIkdJY0ZbdF6xyEHG1gX XXeczGddFlPoQ== Message-ID: <70771a19-9392-4fbb-8aaf-dcd40f60e977@kernel.org> Date: Tue, 20 Oct 2026 09:55:54 +0200 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/memory-tiers: reset demotion targets of nodes without memory To: Gregory Price , Zi Yan Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, joshua.hahnjy@gmail.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, weixugc@google.com, aneesh.kumar@linux.ibm.com References: <20261019184712.3422641-1-gourry@gourry.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/19/26 22:22, Gregory Price wrote: > On Mon, Oct 19, 2026 at 03:04:45PM -0400, Zi Yan wrote: >> On Mon Oct 19, 2026 at 2:47 PM EDT, Gregory Price (Meta) wrote: >>> When a node loses its last memory, offline_pages() clears N_MEMORY >>> before the memory-tier callback rebuilds the demotion targets. >>> disable_all_demotion_targets() resets only N_MEMORY nodes, so the node >>> keeps its old node_demotion[] entry while it has no memory. Before >>> 6c542ab75714, the reset covered every online node. >> >> establish_demotion_targets() uses for_each_node_state(node, N_MEMORY), >> because the to-be-online node is already added into N_MEMORY? >> > > NODE_ADDED_FIRST_MEMORY happens after the memory is already added > (though not necessarily online). > > NODE_REMOVED_LAST_MEMORY happens after the last memory block is removed > (and therefore N_MEMORY is gone). > > slightly different timing for each event. There isn't a reasonable > change we can make otherwise. > >>> >>> A reclaimer reads the stale entry only between the node regaining memory >>> and its targets being rebuilt, and it filters the entry against the >>> current allowed targets, so there is no user-visible effect. To see it, >>> online and then offline a node's only DIMM: a debugger still finds the >>> lower-tier node in node_demotion[nid].preferred. >>> >>> Reset the targets of every node. >>> >>> Fixes: 6c542ab75714 ("mm/demotion: build demotion targets based on explicit memory tiers") >>> Signed-off-by: Gregory Price (Meta) >>> --- >>> mm/memory-tiers.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c >>> index 25e121851b586..7197f1a67a340 100644 >>> --- a/mm/memory-tiers.c >>> +++ b/mm/memory-tiers.c >>> @@ -378,7 +378,7 @@ static void disable_all_demotion_targets(void) >>> struct memory_tier *memtier; >>> int node; >>> >>> - for_each_node_state(node, N_MEMORY) { >>> + for_each_node(node) { >> >> I think the above code deserves a comment explaining why for_each_node() >> is needed instead of for_each_node_state(N_MEMORY), because when a node >> is added or removed during memory hotadd or hotremove matters here. >> > > Sure. Will respin tomorrow and give others a chance to look at it. A comment makes sense to me as well, it's not completely obvious. Apart from that LGTM. -- Cheers, David