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Mon, 28 Sep 2026 08:19:44 -0700 (PDT) Received: from spider.bream-herring.ts.net ([103.6.151.236]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3a0b9feddd0sm7247965a91.2.2026.09.28.08.19.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Sep 2026 08:19:43 -0700 (PDT) From: Matthias Goergens To: Christoph Hellwig Cc: Andrew Morton , Chris Li , Kairui Song , Johannes Weiner , David Hildenbrand , Michal Hocko , Shakeel Butt , Kemeng Shi , Nhat Pham , Yosry Ahmed , Youngjun Park , Baoquan He , Barry Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-api@vger.kernel.org, Alejandro Colomar , linux-man@vger.kernel.org, Karel Zak , util-linux@vger.kernel.org, Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , David Airlie , Simona Vetter , intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, "Rafael J . Wysocki" , Pavel Machek , Catalin Marinas , Will Deacon , linux-pm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Shuah Khan , linux-kselftest@vger.kernel.org Subject: Re: [RFC PATCH v2 0/4] mm/swap: reserve swap areas for deliberate offload Date: Mon, 28 Sep 2026 23:19:34 +0800 Message-ID: <20260928151934.2686418-1-matthias.goergens@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: References: <20260926045517.3458413-1-matthias.goergens@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 8EE36180009 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: 8d8wsnf5w4zhztsuezgm1yx1yds9u3on X-HE-Tag: 1790608785-206389 X-HE-Meta: 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 IRpQJHwj p3Zwdbuz0eZfyy64LsOmLn0Xq99+ZtBrMTmNVWRTl+Wr/9GLpGYOmMd9JWpvoCUsOCH3hWggSb4D0bjpZ/gfYF0iJS4xIZKoAKLOE0rBqvxJYDwuNXbltmUGl5vtJfOkE8v854ud0zxWchccOhP06eItMAOB8EFGvF2LhnRysYGn/rwqkBf7KVM5I89OrD8+ZxMLCQidEYY56pSFHhAWAnTKdbX5wPgEPAeJHZ0L+3xaIW0YYiP2YU+c5FIDjoJLxBtCKsjzXXTSUXPNuGipBLPwjCoEVJo4in988bdI4MDlKnSzxmc9S8ptPNJ8ne2bsk8yrV/z9g3nFNd7XpSXplL9Ec+CbysBoJyVFXG2lhXECqWbMGhgIHhNIBaaYA1Tz3VfSwnrrkDiDsxgnqvNQ5RaXgjA5L/UzP2MKs9C3lgWnifg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Christoph, > Either way block devices must be safe to be called from paging paths, > not just for swap but also for file system based paging. So this is > a bug in the implementation, and not something worked around in the > swap code. Agreed that a backend used for paging has to make progress under pressure, and the zvol example was a poor lead. The series isn't meant to excuse a backend that can deadlock; that would still be a bug to fix in the backend. What I'm after is different: backends that are correct but need memory to accept a write, where the better policy is to use them for cold-page offload when memory isn't tight and keep pressure reclaim on areas that don't need to allocate. Two in-tree examples: zram allocates on writes and fails with -ENOMEM, with no fallback, while its logical size still shows free slots; and filesystem swapfiles must give up copy-on-write, checksums and compression today, because swap writes bypass the filesystem. I'll make that the case for v3 instead. Thanks, Matthias