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Fri, 04 Sep 2026 07:57:14 +0000 X-Mizu-Trace-ID: 85c4279ad02e47cd X-Migadu-Flow: FLOW_OUT Date: Fri, 4 Sep 2026 15:57:11 +0800 From: Baoquan He To: Youngjun Park Cc: Baoquan He , linux-mm@kvack.org, akpm@linux-foundation.org, chrisl@kernel.org, kasong@tencent.com, nphamcs@gmail.com, baohua@kernel.org, hannes@cmpxchg.org, yosry@kernel.org, shikemeng@huaweicloud.com, chengming.zhou@linux.dev, david@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 15/16] mm, swap: require zswap for xswap devices Message-ID: References: <20260827094509.1016740-1-hebaoquan@kylinos.cn> <20260827094509.1016740-16-hebaoquan@kylinos.cn> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 7A61714000C X-Stat-Signature: 9amngscfb7d1ue4oohhsmgypcm3mgyut X-HE-Tag: 1788508636-866075 X-HE-Meta: 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 rJ4RAF4X xMMcc5akNiClFRT6hsZ2i6i5j5zx30qG+eiV9QiIE9Bi5Rd+eoAZvDjhwY+0Tjf2Bee5VT52NWZ2NoapyCAAqt+Wu43vQi+ScH6l+dnG1VDZiuoq0JJcXPfAnQEa5yrIQUzlPwgvgYIHUxoZM3Bp4FvG1DbALkI6sPNuLONMWogUXqTbLdrh8eS+RSx1h7MAxO7+2OwXdweug9I8nWhBocKvf3RkKyJVHt4kNVeCjwKaOuqOztAIBKEW/6KZoAShBWHOnJ/jxOZ5Itig= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09/03/26 at 03:52pm, Youngjun Park wrote: > On Thu, Aug 27, 2026 at 05:45:05PM +0800, Baoquan He wrote: > > xswap has no backing storage: swapped-out pages live only in zswap. > > Without zswap, swapout always bounces back, so the device would > > consume swap entry space without ever freeing memory. Refuse to > > create a device when zswap is unavailable. > > > > Runtime disabling of zswap after creation is safe: existing entries > > stay loadable (zswap_load() gates on zswap_never_enabled(), not the > > runtime zswap_enabled flag) and new swapouts merely bounce back to > > memory without freeing it. > > Hello Boaquan. > > If xswap is used alongside another swap device, wouldn't runtime > disabling of zswap cause a problem? > User assumes other remained swap device used right afte zswap disabled. > > Once a device is allocated as xswap and zswap gets runtime disabled, > xswap keeps receiving new swap entry allocations, and every swapout > to it just bounces back without freeing memory . until xswap's > entries are exhausted. Meanwhile the other swap device sits unused. > > If my point is right... > A few ways to handle this come to mind. > > 1. Once xswap has started accepting writes, refuse to runtime disable zswap. > 2. If zswap is runtime disabled, stop handing out new entries from > xswap (drop it from the available list, or gate on an XSWAP flag). > This would also need nr_swap_pages, the visible swap count to be > reduced accordingly, and unused cluster memory reclaimed. > behave as if xswap had been swapoff'd. > 3. Support falling back to another swap device once zswap becomes > unavailable. Agreed. 2) sounds better. Thanks for the detailed analysis.