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Fri, 21 Nov 2025 03:40:16 -0800 (PST) Received: from localhost ([2620:10d:c092:600::1:610c]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42cb7fd9b45sm10401328f8f.43.2025.11.21.03.40.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 21 Nov 2025 03:40:15 -0800 (PST) Date: Fri, 21 Nov 2025 06:40:11 -0500 From: Johannes Weiner To: Chris Li Cc: Andrew Morton , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , Yosry Ahmed , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org, pratmal@google.com, sweettea@google.com, gthelen@google.com, weixugc@google.com Subject: Re: [PATCH RFC] mm: ghost swapfile support for zswap Message-ID: <20251121114011.GA71307@cmpxchg.org> References: <20251121-ghost-v1-1-cfc0efcf3855@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251121-ghost-v1-1-cfc0efcf3855@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 992A220007 X-Stat-Signature: uqp8c81u9k4f8jxup684rmnkj4xaingq X-HE-Tag: 1763725218-27049 X-HE-Meta: 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 EvpRgpid 9nUmlDHySYE6ALYlEYJyPTDWSv+2NbelFJxoyWyzjkDfskZF97LIiaAcIsv5Z6V4jQHQ8 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 Fri, Nov 21, 2025 at 01:31:43AM -0800, Chris Li wrote: > The current zswap requires a backing swapfile. The swap slot used > by zswap is not able to be used by the swapfile. That waste swapfile > space. > > The ghost swapfile is a swapfile that only contains the swapfile header > for zswap. The swapfile header indicate the size of the swapfile. There > is no swap data section in the ghost swapfile, therefore, no waste of > swapfile space. As such, any write to a ghost swapfile will fail. To > prevents accidental read or write of ghost swapfile, bdev of > swap_info_struct is set to NULL. Ghost swapfile will also set the SSD > flag because there is no rotation disk access when using zswap. Zswap is primarily a compressed cache for real swap on secondary storage. It's indeed quite important that entries currently in zswap don't occupy disk slots; but for a solution to this to be acceptable, it has to work with the primary usecase and support disk writeback. This direction is a dead-end. Please take a look at Nhat's swap virtualization patches. They decouple zswap from disk geometry, while still supporting writeback to an actual backend file. Nacked-by: Johannes Weiner