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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 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 X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: F12C7A0006 X-Stat-Signature: dkbiwmg41jipxbjok5nckund6x8w6g91 X-Rspam-User: X-HE-Tag: 1776109259-657382 X-HE-Meta: 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 euMtPEXs Gb+xhMImcGJ/caGANY0NNBEOosf4h7lFczRcLLrKQ2xnaq1shz36CM2yOYN0/VS7zNF8XxsbuATZMiAhB+/3hXyAFOx0D3+/mrZBi6revMg1Z9aA+iFnLmBS0WHnbNXiCaLpNUU/xLu6b0U8ZPgCNyn9hKEW74lAcLwuA1Ag8IKaOu2dPEkDzNaisSHrq1i2iv++BWvUfriJOYHPgf/z4W5Ct3cQ0F0rGxuzu/l/au3BtF0MHnNdgo6dvHd8h0t6BnCB/ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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