From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lgeamrelo07.lge.com (lgeamrelo07.lge.com [156.147.51.103]) (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 5E1EE391E59 for ; Fri, 12 Jun 2026 07:27:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=156.147.51.103 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781249268; cv=none; b=FzrPU07CU3XrEUTpSt+serXWzQRBou/h/I9xD77nKYhATCJYu8RU+4PbXWq9RfbqOzMtomZNm9xbANNcQhXUyN7DqyIMEylp4NJ1Vj/qtrvlcVnrF/AWZbnITHMKnkivhJO3nqpkyvWN2o7U8S6PW7QKGsAk/eV+OGfrx+x1its= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781249268; c=relaxed/simple; bh=OYWXsPG8UewcUGUYPqG+FW2ak5otVsvhS6Zi21qeS0g=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=SiZRiTKkPXuV+SGJI5cV7wnUP9158HwLT4m/bv+Slh+n5k6YUooV7H6J1DdSkL9EqBbUlvVK3v+ia8LXpvcfbnIuykxq33ScyjiDjHP+d3cTxiLWpB8VIUE/bcql8FID5j+BVzh1C2UT8aGF0Mcuj7rTAqCPv+bbmJCTNY6ZJKo= 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=156.147.51.103 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 156.147.51.103 with ESMTP; 12 Jun 2026 16:27:37 +0900 X-Original-SENDERIP: 10.177.112.156 X-Original-MAILFROM: youngjun.park@lge.com Date: Fri, 12 Jun 2026 16:27:37 +0900 From: YoungJun Park To: Shakeel Butt Cc: Yosry Ahmed , 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: <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 Thu, Jun 11, 2026 at 12:12:40PM -0700, Shakeel Butt wrote: > On Thu, Jun 11, 2026 at 05:45:04PM +0000, Yosry Ahmed wrote: > > On Tue, Jun 09, 2026 at 01:19:13PM +0900, YoungJun Park wrote: > > > 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) > > > > I am not sure if proactive demotion between swap tiers would be driven > > by memory.reclaim, I am guessing a new interface might be more suitable. > > But yes, you are right that it's very possible that > > 'zswap_writeback_only' with memory.reclaim will become obsolete once > > swap tiering matures and starts supporting things like proactive > > demotion. > > > > Part of me wants to wait until the swap tiering interfaces are figured > > out so that we don't end up with redundant interfaces, but I also don't > > want to hold Hao's work since it doesn't directly depend on swap > > tiering. > However I would need zswap folks (Yosry & Nhat) help in figuring out swap tiers > interfaces. Zswap is the current top tier swap usage in real world. I want > zswap users to eaily (and hopefully transparently) migrate to swap tiers. > > Shakeel, how do you want to handle this? I think there's a few options: > > > > 1. Add zswap_writeback_only now, and when we have swap tiering demotion > > it becomes a redundant interface, like memory.zswap.writeback -- or > > maybe we try to deprecate both of them at that point. It's difficult to > > remove interfaces tho, but maybe easier to stop supporting > > zswap_writeback_only. > > > > 2. Add zswap_writeback_only behind an experimental config option, to > > unblock development but have a line of sight to dropping support once we > > have a swap tiering interface. > > > > 3. Wait until we figure out the swap tiering interfaces and then add > > the proactive zswap writeback as part of it. > > > > WDYT? > > Is Hao's work needed for some followup work/development? The earliest Hao's > work can is 7.3, so if we aim to figure out swap tiering interfaces in next > couple of weeks then option 3 is the way to go. If swap tiers take more time > then we can discuss other options as well. > However I would need zswap folks (Yosry & Nhat) help in figuring out swap tiers > interfaces. Zswap is the current top tier swap usage in real world. I want > zswap users to eaily (and hopefully transparently) migrate to swap tiers. I am looking forward to the discussion on this interface! To help boost the discussion and progress, I would like to share a few of my thoughts. We could either introduce a new interface to trigger demotion/promotion, or we could reuse the existing one (using tier just internally) Based on the memcg interface currently proposed in swap_tier (memory.swap.tiers, memory.swap.tiers.effective), I think it aligns well with the current direction. It provides a foundation for selectively targeting devices in tier order. To summarize the discussions so far, the following points align well. - Per-cgroup swap control, as I suggested. - Proactive zswap writeback (Hao's usecase) - Swap device target demotion(if it wants selective, then it is more better), as you mentioned: https://lore.kernel.org/linux-mm/aicZ-5GX9De3MAU7@linux.dev/ - Virtual Swap on/off in the future, as Nhat mentioned: https://lore.kernel.org/linux-mm/20260528212955.1912856-1-nphamcs@gmail.com/ - The memory.zswap.writeback alternative (no hierarchy model conflict) - zswap is first swap tier. - Promotion. (Also better for selectve usage) - tier based swap policy (e.g round-robin...) To accelerate this work, I believe we should reach a consensus and merge the currently proposed swap_tier interface :) If the above approach is difficult, I would like to suggest an alternative for progress with the memcg interfaces removed: 1) We could make zswap the first tier and create a use case where memory.zswap.writeback internally is handled by tier logic. 2) Or simply merge the swap_tier infrastructure itself first. This would allow the swap_tier infrastructure to be merged and discussed more easily. If it takes longer to adopt swap_tier anyway, by doing so we progress next step as a experimental feature. - Apply per-cgroup swap as an experimental (debugfs) feature. - Apply Hao's use case experimentally or as it is as Yosry suggested. (future migration to swap tier) How do you think? (FYI: My emails to kernel.org are failing due to internal server issues.) Thank you Youngjun Park