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Mon, 23 Feb 2026 19:45:05 -0800 (PST) MIME-Version: 1.0 References: In-Reply-To: From: Kairui Song Date: Tue, 24 Feb 2026 11:44:29 +0800 X-Gm-Features: AaiRm53OyQqeexQqNlhOlhsm30QWkdsWCwsnTCjllZxDnHQQ5CWm3htGqwsYvFA Message-ID: Subject: Re: [LSF/MM/BPF TOPIC] Swap status and roadmap discussion To: Yosry Ahmed Cc: Nhat Pham , lsf-pc@lists.linux-foundation.org, Chris Li , YoungJun Park , Barry Song <21cnbao@gmail.com>, Baoquan He , linux-mm , Johannes Weiner Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Queue-Id: 7C9CA4000A X-Rspamd-Server: rspam02 X-Stat-Signature: bmaxe79p5uipugu9nuybq6koazpbhurt X-HE-Tag: 1771904707-830728 X-HE-Meta: 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 RM39sRBg 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Feb 24, 2026 at 2:55=E2=80=AFAM Yosry Ahmed wrot= e: > > > > - Is 64 bits really needed for reverse mapping? For the context, reve= rse > > > mapping here is a swap entry recorded in a lower / physical device > > > pointing to the ghost / virtual device. > > > > I think you can compact this a bit. Swap space itself is not fully 64 > > bits right? > > > > Just not sure if the juice is worth the squeeze to save a couple of > > bits here and there, especially if the reverse mapping is already > > dynamic :) > Hi, thanks for the comment. > I think we should actually revisit the need for a reverse mapping to > begin with. For swapoff, we can probably scan the virtual swap space > looking for entries that belong to the backend being swapped off. Not > as efficient as a reverse map, but still better than the status quo of > scanning page tables. I don't think optimizing for swapoff is worth > the consistent overhead. Right, I don't really think swapoff is worth that much effort too. But there are still ideas like migration and compaction, which could really make use of a proper reverse map. > > The other use cases are probably cluster readahead and swapcache-only > reclaim, and I think both of these can also be revisited. Agree, readahead and swap cache reclaim do need some revisit... Not related to the revert map idea though. I'm thinking if we can make the swap cache completely lazy and never reclaim it proactively for non-RAM swap. And for RAM based swap (zswap / ZRAM), do the opposite, always ensure swap cache is reclaimed after use.