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 AC2173655CF for ; Tue, 9 Jun 2026 04:34:14 +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=1780979658; cv=none; b=Aev3ZwDSERk8bUUrvFEZhlYHbOEbVFDBKDDf/pddqAn8S3dAYrruSur/mycfmqdECScK+WCk1ZilJnOGEHhPGQX68MgQ2tQKgFnWtiQ8/c3w2D1irmdOi29ct4LH4OXGuto1uwJS5C59i84YymcjuKuFwaUmfrU/HLKEWDHShQE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780979658; c=relaxed/simple; bh=+hQ+RWuqaS/Jpfm/1pJ71r7akfcY/Ua/eGAmRinOwPI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ftpHvTkLg07IWbgs2rh9eyJGAsiAPl9Fpgp209T70DC9aVu+i6fCU7/6Zh74ZBY+yDkGeLErXNjYfu3JHAjJ8O14zQ+U/svqSISrXRQs3gLxr6wWNulFK5tSVy3+6dAvxD4oMrZYfMeqMy8iLFVwL5asJeyya5xuorSyfhsvvAA= 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) (10.177.112.156) by 156.147.51.102 with ESMTP; 9 Jun 2026 13:19:13 +0900 X-Original-SENDERIP: 10.177.112.156 X-Original-MAILFROM: youngjun.park@lge.com Date: Tue, 9 Jun 2026 13:19:13 +0900 From: YoungJun Park To: Yosry Ahmed Cc: Shakeel Butt , Hao Jia , Johannes Weiner , mhocko@kernel.org, tj@kernel.org, mkoutny@suse.com, roman.gushchin@linux.dev, Nhat Pham , akpm@linux-foundation.org, chengming.zhou@linux.dev, muchun.song@linux.dev, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Hao Jia , chrisl@kernel.org, kasong@tencent.com, baoquan.he@linux.dev Subject: Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback Message-ID: References: <6deeaea7-3cd1-4403-29fc-d2dc55c297f8@gmail.com> <6db27a22-cc7a-9a94-db3f-c912fd39aa32@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jun 08, 2026 at 03:27:07PM -0700, Yosry Ahmed wrote: +Chris +Kairui +Baoquan Hello Thanks for inviting me to the discussion, Shakeel. > > > > Youngjun is working on swap tiers. At the moment he is more interested in > > > > allowing a specific swap device to a memcg or not. I can imagine in future there > > > > will be use-cases where there will be a need to demote data on higher tier swap > > > > to lower tier swap. What would be the appropriate interface? Speaking of my work on swap tiers, I recently submitted a patch and am currently considering memcg integration: https://lore.kernel.org/linux-mm/20260527062247.3440692-1-youngjun.park@lge.com/ The future use-cases imagined above seem to align with this direction. (BTW, I am currently waiting for reviews/feedback from the memcg folks on this patch. Any reviews would be highly appreciated!) We could potentially assign a target tier for writeback within the existing memory.zswap.writeback interface. For instance, '0' could mean disabled, while non-zero values could represent specific tiers, which would maintain backward compatibility with the current version. Alternatively, if zswap is treated as the default top tier, the `memory.swap.tiers` interface could potentially replace `memory.zswap.writeback`. Furthermore, this could be expanded so that each swap tier can demote data user-triggered demotion between swap tiers. Based on the current patch's ideas combined with my swap tiers concept: Assuming a hierarchy like: zswap -> tier1 (SSD swap) -> tier2 (HDD swap) -> tier3 (Network swap) We could configure the active tiers via a setting like `memory.swap.tiers` (tier2 enabled, tier3 enabled). For example, the concept of `echo "100M zswap_writeback_only > memory.reclaim"` could be extended. A user could run `echo "100M tier2 > memory.reclaim"` to explicitly trigger demotion from tier2 to tier3. (BTW, if we combine these features, my personal preference for the keyword format would be ` `. I think it would be better to explicitly indicate that it is a swap demotion by using a specific prefix followed by the tier name. Or make demote prefix another key is also possible) So, the whole picture would look something like this: - memory.swap.tiers : Interface for configuring the tier mask. - memory.reclaim : Entry point for user-triggered demotion. > > > Things will probably get more > > > blurry with memory tiers and compressed memory nodes though. > > > > I think there will still be distinction between byte addressable and fault on > > access devices. > > Yeah, I think it makes sense to define "swap" as fault on access > (zswap, SSD, etc), and memory tiers as byte-addressable (even if you > put an SSD behind CXL and make it byte-addressable). But I also > remember seeing discussions about unifying memory tiers and swap in a > way, and it makes sense from a reclaim perspective (swap or demote > first?). > > > > will be use-cases where there will be a need to demote data on higher tier swap > > > > to lower tier swap. What would be the appropriate interface? > > > > > > > > BTW does zswap folks think of zswap as a top swap tier or something different? > > > > > > I haven't been following the swap tiers work closely, but personally I > > > do think of zswap as a top swap tier. Regarding zswap's position, I agree it needs to be defined as the default, top-most tier in swap_tier. In my early RFC, I allocated a separate tier specifically for zswap: (https://lore.kernel.org/linux-mm/20251109124947.1101520-3-youngjun.park@lge.com/) > > Same for me though I imagine swap tiers would introduce some duplication i.e. > > different way (interface) to set limits for swap tiers for a given memcg. > > I also agree with the concern about interface duplication. We will eventually need a mechanism to control swap amounts per tier, which requires thinking about its relationship with swap.max. (I raised this as an open question in my early RFC). https://lore.kernel.org/linux-mm/20251109124947.1101520-1-youngjun.park@lge.com/ (Further Discussion and Open Questions Part) However, since this feature is necessary anyway, wouldn't the proposed interface be acceptable without causing conflicts at this early stage? Additionally, `memory.zswap.writeback` seems redundant. Restricting a cgroup to only use the zswap tier (assuming it's the first tier) is practically identical to disabling `memory.zswap.writeback` (correct me if I'm wrong). But there is no problem to integrate it as I think e.g `memory.zswap.writeback` could internally act as an alias for setting `memory.swap.tier` to 'zswap only'. BR, Youngjun Park