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From: Baoquan He To: Nhat Pham Cc: linux-mm@kvack.org, akpm@linux-foundation.org, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, baohua@kernel.org, youngjun.park@lge.com, hannes@cmpxchg.org, yosry@kernel.org, chengming.zhou@linux.dev, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 00/10] mm/swap: ghost swapfile with backend switching via Redirect entries Message-ID: References: <20260707082614.95030-1-baoquan.he@linux.dev> 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 07/15/26 at 07:21pm, Baoquan He wrote: > Hi Nhat, > > On 07/12/26 at 05:02pm, Nhat Pham wrote: > > On Thu, Jul 9, 2026 at 4:39 AM Baoquan He wrote: > ...snip... > > Sorry, I won't reply to each comment individually. I'll just leave > a summary reply at the end. > > > > I think VSS goes beyond solving the "zswap without backing store" > > > problem — it is fundamentally an architectural restructuring of the > > > swap subsystem, with virtual swap as a first-class abstraction layered > > > above physical devices. That is both its strength (comprehensive, > > > future-proof) and its trade-off (more invasive to existing paths). > > > I respect that. > > > > > > My goal with ghost swap is different: address the immediate pain point > > > with the smallest possible change to the existing infrastructure. Both > > > approaches have value, and I hope the community discussion will help > > > identify the right balance. > > > > That's because I believe "zswap without backing store" just by itself > > is a bit narrow of a problem. It's certainly nice to fix, but it's > > more of a nuisance - you already have userspace hacks for it (as I > > mentioned above). > > > > The problem I'm trying to solve is to support: > > > > 1. Writeback. > > > > 2. Dynamicity. > > > > 3. Decoupled backends. > > > > all of which are motivated by real production issues, not some > > theoretical problems. I'm concerned that if we only focus on the third > > goal, we'll dig ourself into a hole that prevents us from solving 1 > > and 2 efficiently down the line. > > > > > > > > Thank you again for the detailed review — it has already clarified > > > which parts of the design need more thought and documentation, and will > > > make the next version stronger. > > > > I really don't want to be antagonistic, but I hope you'd take the real > > production pains that we've had (and have tried to communicate in > > multiple mailing threads, across a timeline of almost 2 years at this > > point) seriously. > > > > Collaborations should go both ways. I've gone out of my way to try to > > address the concerns of various parties, from spending *multiple > > weeks* testing and investigating performance regression on zram > > backend (which my company does not use), to a rewrite/re-design of > > virtual swap to accommodate parties who wished to opt out of virtual > > swap for now. > > > > I hope you can extend the same good will to our needs :) I've included > > you (and other swap folks such as Chris and Kairui) in my cc-lists. If > > you have concerns, you could have commented. Instead, you decided to > > send a patch series, which is basically just the ghost swapfile, with > > a bit of afterthought to handle writeback and dynamicity, rather > > inefficiently (no dedicated per-CPU caching), and not even correctly > > (the lack of rmap means swapoff / swap-cache-only physical swap slot > > reclaim is broken). > > > > I respect you and Chris very much to assume bad faith, but please work > > with me rather than against me. > > Before I get into the technical discussion, I want to address something > you raised at the end of your reply — the concern that I'm working > against you rather than with you. Our paths in MM simply haven't crossed > much until now. I can't think of reason why I would want to target you > or your work. > > I respect the work you've put into VSS. I did check it. Honestly I am not > fan of it. In your v2: 2244 insertions, 250 deletions, 15 files touched, > a new 455-line header (vswap.h). While swap subsystem has just absorbed the > swap table series. Then another 2200 lines of architectural restructuring > on top is added. Not sure if anybody raise concern about your solution, > or anybody suggests other directions. > > I posted ghost swap because I saw a concrete, narrow problem that I believed > could be solved with a small change. It makes decoupling and dynamic > growth with the minimum possible mechanism. Writeback is explicitly deferred > — not because it's unimportant, but because it's a separate problem that > shouldn't block the common case. With my shallow knowledge, it's optimal > solution to the encountered problem. If you agree, you can change to Part of above sentence is missing. I meant if you agree on the ghost solution, you can change to take its way and continue, just add me to CC. Or we can work together. > Otherwise, please continue to push virtual swap forward. I'll stop and > wait. > > Thanks > Baoquan