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From: Shakeel Butt To: Yosry Ahmed Cc: 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 , youngjun.park@lge.com 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT On Mon, Jun 08, 2026 at 01:19:32PM -0700, Yosry Ahmed wrote: > On Mon, Jun 8, 2026 at 12:50 PM Shakeel Butt wrote: > > > > +Youngjun > > > > On Mon, Jun 08, 2026 at 11:30:30AM -0700, Shakeel Butt wrote: > > > On Wed, Jun 03, 2026 at 10:36:07PM -0700, Yosry Ahmed wrote: > > > > > >> But doesn't it make more sense to specify the compressed size, which is > > > > > >> ultimately the amount of memory you actually want to reclaim. > > > > > >> > > > > > > > > > > > > I personally prefer compressed size to pre-compressed size. That's > > > > > > kinda what user cares about, no? > > > > > > > > > > > > One thing we can do is let users prescribe a compressed size, but > > > > > > internally, we can multiply that by the average compression ratio. > > > > > > That gives us a guesstimate of how many pages we need to reclaim, and > > > > > > you can follow the rest of your implementation as is (perhaps with > > > > > > short-circuit when we reach the goal with fewer pages reclaimed). > > > > > > > > > > Got it. I will change it to use the compressed size in the next version. > > > > > > > > > > Yosry, Nhat, should we continue using the zswap_writeback_only key to > > > > > trigger proactive writeback? > > > > > > > > I *really* want the memcg maintainers to chime in here, it's > > > > ultimately their call. > > > > > > > > Michal? Johannes? Shakeel? Roman? Anyone? :D > > > > > > Between the options of having an explicit interface (i.e. > > > memory.zswap.writeback*) or a key (i.e. zswap_writeback_only) to memory.reclaim > > > interface, I prefer the key option. I have not looked into how much proactively > > > reclaiming zswap memory or proactively triggering zswap writeback makes sense > > > but from the perspective of memcg interface, I think the key option would give a > > > more clean solution if we decide in the future that this whole thing was a bad > > > idea. > > > > > > Next regarding future proofing zswap writeback trigger, do we expect any > > > potential additions/changes/new-features for this interface? For example do we > > > expect in future we may want to trigger the zswap writeback only from a specific > > > node or lowest memory tier? > > The way I see it, zswap writeback is just a "special" type of > proactive reclaim, but the goal is still proactively freeing cold > memory. In that regard, I think it makes sense to have things like > node-specific reclaim. Not sure about other extensions, but Hao > initially suggested making this age-based, so I think the answer is > yes. > > For both of these examples (node-specific reclaim, age-based reclaim), > I think the same semantics could apply to memory.reclaim in general, > which is why I suggested making it a part of memory.reclaim. I also > like the idea of having a single proactive reclaim interface in > general, but maybe we don't want to overload it too much. IMHO it is fine. > > > 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? > > > > 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. 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. > 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.