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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 Subject: Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback Message-ID: References: <20260526114601.67041-1-jiahao.kernel@gmail.com> <20260526114601.67041-3-jiahao.kernel@gmail.com> <6deeaea7-3cd1-4403-29fc-d2dc55c297f8@gmail.com> <6db27a22-cc7a-9a94-db3f-c912fd39aa32@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 84A9E80014 X-Stat-Signature: iwouq3t7q8zftj9mbttkyqo3jndcaasq X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1780943439-582097 X-HE-Meta: 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 BKa7yTwA 0iyBksOseyIE0Nj3EoJB7AGfDlakNdIhzGZW01ljybU8TsQ/93DamKF8POyL5RhhMy76QKqARSlXluCJRd48JkKCBl+gBBGmCKsjNrnTSZjJl3oqO5PU3cuAgEHgqS9W5Nj3zgecMSEZKBr4XSgfe1xrSPF1VXeaIJYUSKlJfgLWBhY6X5ETZraC6jd1d9nsogDJHd/hbk7XJIA9v2vfG/1p//W+Lgg4mbyIpR1XsFfBnw+9BSaz4TWnO2u6kL0hlajjCmb+oB0Vlif+zMgfNuTes7VoV1SQKYpeV Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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?