From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6612D38F926; Mon, 13 Apr 2026 19:40:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776109259; cv=none; b=rUI7P3DkNg1hMSVQph+aBMCX67tMNvRFzbkv9xdx2lLeBKlKo9jHU/PAs1hRFurtV4zSc2339XhHWH8ilQ1Socsq/pXS+4/dSi333VSnx+1XflqCyqJpSMs9AHtIcIqUAkf4PPxIh8Wo+8wYN+nRo3QKAOu5lK3hvaFuFVIckaw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776109259; c=relaxed/simple; bh=3n/Uu2a1dKK5ibn+zUIP0gw654NIOknL9hOGX53p6nk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=t+TDA74KpuRmZBtLwSVsqnNvsOD60sSMg+HdbRHg1RDZxDL1L0M7Ar69gIDc2+/0GuJx4AhNhRVRkugcQQ7P9HFqJcKt6GIHCsM63p4kOB7Y4fv4zrRGH1jwB1G9E03+FlrHb58fz+7fMHjB8W7ZPtijiqHrSjXSFoLhoyGjNhE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JgZtJK/6; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JgZtJK/6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 08830C2BCAF; Mon, 13 Apr 2026 19:40:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776109259; bh=3n/Uu2a1dKK5ibn+zUIP0gw654NIOknL9hOGX53p6nk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=JgZtJK/6tu56BCfWrlRneew2guuh2eizHFp4Rfv23WjcuVKaTM6hJMBiXUp40FI3T I7/Rf7QAkINChRQF+GmJZClTXjCLp42W88u9189PbN0l9Y90h08IMkAkqlBY6Vc/Bb VaVEW2LslBew4Zsnfs/MHbn8C8oq6dqyjC3EOPYgszHug2EIzuEOex2/xo3a8yb22o 5CbtoZWKoHB6b8UGqZlcrvkK2OfkOynlnwm9dIPzNgXLU08/SdEjP0GmUuCAW/tb6e WwrJLNL1gehBHi+IDDIJ/EsykzS3ounIQiI/FH3vyHp/M+FWSmjqlKLmK1yXuRjwh9 C73lJeO701WhQ== Message-ID: <478c5700-8bf5-42b9-ad71-bbc618b36c9c@kernel.org> Date: Mon, 13 Apr 2026 21:40:49 +0200 Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/8] dax/kmem: atomic whole-device hotplug via sysfs To: Gregory Price Cc: Andrew Morton , linux-mm@kvack.org, vishal.l.verma@intel.com, dave.jiang@intel.com, osalvador@suse.de, dan.j.williams@intel.com, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-kernel@vger.kernel.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, kernel-team@meta.com References: <20260321150404.3288786-1-gourry@gourry.net> <20260321104021.4a6074330131a2058e8706bd@linux-foundation.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/13/26 21:38, Gregory Price wrote: > On Mon, Apr 13, 2026 at 05:47:01PM +0200, David Hildenbrand (Arm) wrote: >> >> So, really only user space can try offlining the memory after requested >> onlining succeeded. >> >> I don't think any udev rules do that? The usually only request to >> online, which should be fine. >> > > In the offline case the block will cease to exist after offlin/remove > returns, so what should happen is the race on offline just fails and > the stale object cleans itself up on the way out after failure. > > Userland temporarily sees a stale memory block but can't do anything > with it because sync'd on hotplug lock. Exactly. > >> So if a user does that manually, good for him. We just have to make sure >> that stuff keeps working as expected. >> > > Yeah the only catch is if a user does something dumb like > > cat block/state -> online_movable > echo offline > block/state > echo online > block/state > > But like... don't do that :] Yes :) > > Udev won't ever offline-online-race like this, so it's not a real issue. > >> Or am I missing a case? >> > > So yeah, I'm fairly confident this just works. > >> >> I'll note that offline_and_remove_memory() can take a long time/forever >> to succeed. User space can abort it by sending a critical signal. >> >> For example, if you do >> >> $ echo "unplugged" > magic_device_file >> >> And it hangs, user space can kill the "echo" command, sending a fatal >> signal and making offline_and_remove_memory() fail. >> >> The question is, if you want to do your best to revert the other offline >> operations and try re-adding/onlining what you already offlined. >> >> offline_and_remove_memory() handles that much nicer internally, as it >> tries to revert offlining, and only removes once everything was offlined. >> >> I think I raised it previously, but you could add a >> offline_and_remove_memory_ranges() that consumes multiple ranges, and >> would do this for you under a single lock_device_hotplug(). >> > > I don't think this is a very large lift, just a slightly larger hotplug > locking scope. But then - the per-range thing in this set should just > work, so let me know if it's worth the extra churn. Well, you can rather cleanly undo the operation if any offlining fails. If that's not a requirement, then no need to change it! -- Cheers, David