From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1A88EB64DD for ; Tue, 27 Jun 2023 15:14:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232201AbjF0PO1 (ORCPT ); Tue, 27 Jun 2023 11:14:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38930 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232141AbjF0POY (ORCPT ); Tue, 27 Jun 2023 11:14:24 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 89F0926AB for ; Tue, 27 Jun 2023 08:14:22 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 372552188A; Tue, 27 Jun 2023 15:14:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1687878861; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=q+D0xhAl6bCP8w4Ev6dkVwB38CzSCevMrsveKQ90aLY=; b=HNdgg5a0G7+6HiDl5KyxLX0wrgSDAiYzCIlkI9EAqxSI+pCAwVpQ12whuebX5nXV8uXt3s u50x/bMzzJBPKP/j3fRw8VBZ+GNYaItLYsoiygmHQXAIhTwfpgZU/bKdS6VcEMuwSsBVUM gWJwKULODMlTs634Ad/ljD6V92ZN7TA= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 1A56713276; Tue, 27 Jun 2023 15:14:21 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id M3OWA838mmQzJQAAMHmgww (envelope-from ); Tue, 27 Jun 2023 15:14:21 +0000 Date: Tue, 27 Jun 2023 17:14:20 +0200 From: Michal Hocko To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, virtualization@lists.linux-foundation.org, Andrew Morton , "Michael S. Tsirkin" , John Hubbard , Oscar Salvador , Jason Wang , Xuan Zhuo Subject: Re: [PATCH v1 3/5] mm/memory_hotplug: make offline_and_remove_memory() timeout instead of failing on fatal signals Message-ID: References: <20230627112220.229240-1-david@redhat.com> <20230627112220.229240-4-david@redhat.com> <74cbbdd3-5a05-25b1-3f81-2fd47e089ac3@redhat.com> <0929f4b9-bdad-bcb4-4192-44e88378016b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0929f4b9-bdad-bcb4-4192-44e88378016b@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 27-06-23 16:57:53, David Hildenbrand wrote: > On 27.06.23 16:17, Michal Hocko wrote: > > On Tue 27-06-23 15:14:11, David Hildenbrand wrote: > > > On 27.06.23 14:40, Michal Hocko wrote: > > > > On Tue 27-06-23 13:22:18, David Hildenbrand wrote: > > > > > John Hubbard writes [1]: > > > > > > > > > > Some device drivers add memory to the system via memory hotplug. > > > > > When the driver is unloaded, that memory is hot-unplugged. > > > > > > > > > > However, memory hot unplug can fail. And these days, it fails a > > > > > little too easily, with respect to the above case. Specifically, if > > > > > a signal is pending on the process, hot unplug fails. > > > > > > > > > > [...] > > > > > > > > > > So in this case, other things (unmovable pages, un-splittable huge > > > > > pages) can also cause the above problem. However, those are > > > > > demonstrably less common than simply having a pending signal. I've > > > > > got bug reports from users who can trivially reproduce this by > > > > > killing their process with a "kill -9", for example. > > > > > > > > This looks like a bug of the said driver no? If the tear down process is > > > > killed it could very well happen right before offlining so you end up in > > > > the very same state. Or what am I missing? > > > > > > IIUC (John can correct me if I am wrong): > > > > > > 1) The process holds the device node open > > > 2) The process gets killed or quits > > > 3) As the process gets torn down, it closes the device node > > > 4) Closing the device node results in the driver removing the device and > > > calling offline_and_remove_memory() > > > > > > So it's not a "tear down process" that triggers that offlining_removal > > > somehow explicitly, it's just a side-product of it letting go of the device > > > node as the process gets torn down. > > > > Isn't that just fragile? The operation might fail for other reasons. Why > > cannot there be a hold on the resource to control the tear down > > explicitly? > > I'll let John comment on that. But from what I understood, in most setups > where ZONE_MOVABLE gets used for hotplugged memory > offline_and_remove_memory() succeeds and allows for reusing the device later > without a reboot. > > For the cases where it doesn't work, a reboot is required. Then the solution should be really robust and means to handle the failure - e.g. by retrying or alerting the admin. > > > > > Especially with ZONE_MOVABLE, offlining is supposed to work in most > > > > > cases when offlining actually hotplugged (not boot) memory, and only fail > > > > > in rare corner cases (e.g., some driver holds a reference to a page in > > > > > ZONE_MOVABLE, turning it unmovable). > > > > > > > > > > In these corner cases we really don't want to be stuck forever in > > > > > offline_and_remove_memory(). But in the general cases, we really want to > > > > > do our best to make memory offlining succeed -- in a reasonable > > > > > timeframe. > > > > > > > > > > Reliably failing in the described case when there is a fatal signal pending > > > > > is sub-optimal. The pending signal check is mostly only relevant when user > > > > > space explicitly triggers offlining of memory using sysfs device attributes > > > > > ("state" or "online" attribute), but not when coming via > > > > > offline_and_remove_memory(). > > > > > > > > > > So let's use a timer instead and ignore fatal signals, because they are > > > > > not really expressive for offline_and_remove_memory() users. Let's default > > > > > to 30 seconds if no timeout was specified, and limit the timeout to 120 > > > > > seconds. > > > > > > > > I really hate having timeouts back. They just proven to be hard to get > > > > right and it is essentially a policy implemented in the kernel. They > > > > simply do not belong to the kernel space IMHO. > > > > > > As much as I agree with you in terms of offlining triggered from user space > > > (e.g., write "state" or "online" attribute) where user-space is actually in > > > charge and can do something reasonable (timeout, retry, whatever), in these > > > the offline_and_remove_memory() case it's the driver that wants a > > > best-effort memory offlining+removal. > > > > > > If it times out, virtio-mem will simply try another block or retry later. > > > Right now, it could get stuck forever in offline_and_remove_memory(), which > > > is obviously "not great". Fortunately, for virtio-mem it's configurable and > > > we use the alloc_contig_range()-method for now as default. > > > > It seems that offline_and_remove_memory is using a wrong operation then. > > If it wants an opportunistic offlining with some sort of policy. Timeout > > might be just one policy to use but failure mode or a retry count might > > be a better fit for some users. So rather than (ab)using offline_pages, > > would be make more sense to extract basic offlining steps and allow > > drivers like virtio-mem to reuse them and define their own policy? > > virtio-mem, in default operation, does that: use alloc_contig_range() to > logically unplug ("fake offline") that memory and then just trigger > offline_and_remove_memory() to make it "officially offline". > > In that mode, offline_and_remove_memory() cannot really timeout and is > almost always going to succeed (except memory notifiers and some hugetlb > dissolving). > > Right now we also allow the admin to configure ordinary offlining directly > (without prior fake offlining) when bigger memory blocks are used: > offline_pages() is more reliable than alloc_contig_range(), for example, > because it disables the PCP and the LRU cache, and retries more often (well, > unfortunately then also forever). It has a higher chance of succeeding > especially when bigger blocks of memory are offlined+removed. > > Maybe we should make the alloc_contig_range()-based mechanism more > configurable and make it the only mode in virtio-mem, such that we don't > have to mess with offline_and_remove_memory() endless loops -- at least for > virtio-mem. Yes, that sounds better than hooking up into offline_pages the way this patch is doing. -- Michal Hocko SUSE Labs