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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id af79cd13be357-93bedb05a2esm213660485a.33.2026.09.19.09.08.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 19 Sep 2026 09:08:13 -0700 (PDT) Date: Sat, 19 Sep 2026 12:08:11 -0400 From: Gregory Price To: Chris Li Cc: Johannes Weiner , Baoquan He , Nhat Pham , Kairui Song , Michal Hocko , Roman Gushchin , Shakeel Butt , Yosry Ahmed , David Hildenbrand , Muchun Song , Kemeng Shi , Barry Song , YoungJun Park , Chengming Zhou , "Lorenzo Stoakes (Oracle)" , "Liam R. Howlett" , "Vlastimil Babka (SUSE)" , Mike Rapoport , Suren =?utf-8?B?QmFnaGRhc2FyeWFu77+8?= , Qi Zheng , Axel Rasmussen , Yuanchu Xie , Wei Xu , Rik van Riel , Wenchao Hao , Jonathan Corbet , Hugh Dickins , Baolin Wang , Tejun Heo , Michal =?utf-8?Q?Koutn=C3=BD?= , Shuah Khan , Kunwu Chan , Meta kernel team , Linux Memory Management List , Linux Kernel Mailing List , linux-doc@vger.kernel.org, "open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" , Andrew Morton , Kairui Song , Joshua Hahn Subject: Re: Path forward for Virtualized Swap? Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: 2B6CC1C0002 X-Stat-Signature: tmurtd5z673d8sm66zxnagxe7j3qszup X-Rspamd-Server: rspam01 X-HE-Tag: 1789834095-774939 X-HE-Meta: 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 I+q7yJho NcP9JHnK/A01d0Z1p9K7keSg7gytBoow1tefBoaEyt5l5Abr8FxzSsGJIr1EO/fKdhq2iTkUQIg3FnGM4aLeoeAp1rDksbOjUOpi3nlXOKJyca7iZ7KXLbkVlUqoPwsLf3KRyF48hDqnb4f2KctFCejta8FvuX2UaVN4dZTgHlFINY5JHBXULlBaeC8WSis7cwu0vt9/3c5sz4R0q5B2F7K0aTaDPiuL8T7rCdOJMpDJAVijbqQFVldu0pNSTwkMsbZGIAfCWGPz/KvL9tbMCBfF+QLp9V0VwKz19BYRLu8c3+A3sOZERevd/koqSFuqItGiwC/8BqoHu/dKFu/tKuZYnXYRs2g7aaiVVrcas2HeL6NNzlZL7XavRO5ZEMXRuvLa8+33tmboic79IbKIh1iQukhpDaqjev3ng2CY/tnt4OYJGwM+9OoioPZNZ9GpVGCxV Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, Sep 19, 2026 at 03:45:03AM -0500, Chris Li wrote: > > I found this new concept of "compression space" very confusing to me. > Can you explain the swap behavior and problem using only normal memory > usage reduction and latency without introducing a new term or new > metrics? > > The normal user doesn't even know what compression space is, let alone > what makes it transparent. > Sure they do - it's the amount of memory consumed by compressed data, including the metadata associated with it. converting Johannes statement to diagram: >> Compression space is not a separate resource. It's page tables, >> backing pages, and swap descriptors. It's just MEMORY. Page Data (PD) [page tables][ uncompressed page ] Compressed Data (CD) [ recovered space ][pte][swap meta data][compressed page] | | |---------compression space----------| Memory Pre-Compression |[ PD ][ PD ][ PD ][ PD ][ PD ][ PD ][ PD ][ PD ]| Memory Post-Compression |[CD][CD][CD][CD][CD][CD][CD][CD]-------- free space ------------| ^----------------------------^ Compression Space It's actually really confusing to represent this space as a traditional swap device - built on the assumption of a pre-defined size limit - when that size limit has already been defined (the memory itself). Baking any kind of pre-defined limit *into the infrastructure* (whether for an optimization or ease of implementation) is what requires users to know what compression space is. Even using memory capacity at boot as a limit is wrong - hotplug exists. > Why does the user need to care about compression space again? They shouldn't need to, that's very much the point. ~Gregory