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From: Shakeel Butt To: Yosry Ahmed Cc: YoungJun Park , 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: cgroups@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: X-Migadu-Flow: FLOW_OUT 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. > > 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.