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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768388127; bh=hIuzYS6a+N3lbpi7hIrLjsDBa5QpNFERvUevfsOEfh4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=UFqFErqzPJ9xHsMZc9dlgwx8hnhxKPXXPYUA7gEGkj+uw1ZDtSkdXfwehvBaAUol6 /waiG99gkQrkadIv8oSCIckIZblqs4CzJfSq1HCXsQYuoX/lPqgirgy1JXbyvF2jMp yls+m+cGqpmVjJ1+5oLRmxDTJx4caD6F6XcJxGkgXJZqBk4sKs0U/zCZW+nlvJD3dc 2TmzI4xtK2riCIBJCO7FIdzuy5ipOvxHnUH3EiP66tjgWs7KYS2vxMt0pNJ2EbXS5q CC2/9ogYjJtlFKGXCK6PAXDRVwgYd8IjzO3GXNqhfcRjF0tZ/1NnTW+EtjPa22nxTZ Z0gLImtzc6lZA== Message-ID: <3555385d-23de-492c-8192-a991f91d4343@kernel.org> Date: Wed, 14 Jan 2026 11:55:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 7/8] dax/kmem: add sysfs interface for runtime hotplug state control To: Gregory Price , linux-mm@kvack.org Cc: linux-cxl@vger.kernel.org, nvdimm@lists.linux.dev, linux-kernel@vger.kernel.org, virtualization@lists.linux.dev, kernel-team@meta.com, dan.j.williams@intel.com, vishal.l.verma@intel.com, dave.jiang@intel.com, mst@redhat.com, jasowang@redhat.com, xuanzhuo@linux.alibaba.com, eperezma@redhat.com, osalvador@suse.de, akpm@linux-foundation.org References: <20260114085201.3222597-1-gourry@gourry.net> <20260114085201.3222597-8-gourry@gourry.net> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: j61xe1pkeydo5o46p6heqkyejetswmop X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 5FEE280007 X-HE-Tag: 1768388128-923316 X-HE-Meta: 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 H97OoktH kNcLj3k/1zcrzb15P9TEjwkOeoBp5pD/P12nvc8Box3x1HVGDejIju+8M3VWZjRt/oLoxYoRHamI9kYJl6Lo43Up7UbPeZT9+2m/P1oSRN029uKN7++4CcwaFw7f6GHu5dFeR5BNVidJIbYMFFekRSRBzt4SeFq5n5lAyVgIBgs8j+mdf/17YUCJ5I0OGKsH1wtSVCL+gFgW3EM2DeCYLju7NsyG7J/QW0iTU0c1VMWaHHgnT7/Dng3L8nfNKQZguUfE2 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 1/14/26 09:51, Gregory Price wrote: > The dax kmem driver currently onlines memory automatically during > probe using the system's default online policy but provides no way > to control or query the memory state at runtime. Users cannot change > the online type after probe, and there's no atomic way to offline and > remove memory blocks together. > > Add a new 'hotplug' sysfs attribute that allows userspace to control > and query the memory state. The interface supports the following states: > > - "offline": memory is added but not online > - "online": memory is online as normal system RAM > - "online_movable": memory is online in ZONE_MOVABLE > - "unplug": memory is offlined and removed > > The initial state after probe uses MMOP_SYSTEM_DEFAULT to preserve > backwards compatibility - existing systems with auto-online policies > will continue to work as before. > > The state machine enforces valid transitions: > - From offline: can transition to online, online_movable, or unplug > - From online/online_movable: can transition to offline or unplug > - Cannot switch directly between online and online_movable Do we have to support these transitions right from the start? What are the use cases for adding memory as offline and then onlining it, and why do we have to support that through this interface? It would be a lot simpler if we would only allow > - "offline": memory is added but not online > - "online": memory is online as normal system RAM > - "online_movable": memory is online in ZONE_MOVABLE > - "unplug": memory is offlined and removed That is, transitioning from offline to online or vice versa fails with -ENOSUPP. User space can do that itself through sysfs and if there is ever a good use case we can extend this interface here to allow it. Or is there a good use case that really requires this? -- Cheers David