From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f200.google.com (mail-qk1-f200.google.com [209.85.222.200]) by kanga.kvack.org (Postfix) with ESMTP id BA87B6B2054 for ; Tue, 20 Nov 2018 08:58:16 -0500 (EST) Received: by mail-qk1-f200.google.com with SMTP id a199so3377375qkb.23 for ; Tue, 20 Nov 2018 05:58:16 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id q4si5493813qtb.265.2018.11.20.05.58.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 20 Nov 2018 05:58:15 -0800 (PST) Date: Tue, 20 Nov 2018 21:58:03 +0800 From: Baoquan He Subject: Re: Memory hotplug softlock issue Message-ID: <20181120135803.GA3369@MiWiFi-R3L-srv> References: <20181119105202.GE18471@MiWiFi-R3L-srv> <20181119124033.GJ22247@dhcp22.suse.cz> <20181119125121.GK22247@dhcp22.suse.cz> <20181119141016.GO22247@dhcp22.suse.cz> <20181119173312.GV22247@dhcp22.suse.cz> <20181119205907.GW22247@dhcp22.suse.cz> <20181120015644.GA5727@MiWiFi-R3L-srv> <3f1a82a8-f2aa-ac5e-e6a8-057256162321@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3f1a82a8-f2aa-ac5e-e6a8-057256162321@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins , Michal Hocko , Vlastimil Babka , pifang@redhat.com Cc: David Hildenbrand , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, aarcange@redhat.com, Mel Gorman Hi, On 11/20/18 at 02:38pm, Vlastimil Babka wrote: > On 11/20/18 6:44 AM, Hugh Dickins wrote: > > [PATCH] mm: put_and_wait_on_page_locked() while page is migrated > > > > We have all assumed that it is essential to hold a page reference while > > waiting on a page lock: partly to guarantee that there is still a struct > > page when MEMORY_HOTREMOVE is configured, but also to protect against > > reuse of the struct page going to someone who then holds the page locked > > indefinitely, when the waiter can reasonably expect timely unlocking. > > > > But in fact, so long as wait_on_page_bit_common() does the put_page(), > > and is careful not to rely on struct page contents thereafter, there is > > no need to hold a reference to the page while waiting on it. That does > > So there's still a moment where refcount is elevated, but hopefully > short enough, right? Let's see if it survives Baoquan's stress testing. Yes, I applied Hugh's patch 8 hours ago, then our QE Ping operated on that machine, after many times of hot removing/adding, the endless looping during mirgrating is not seen any more. The test result for Hugh's patch is positive. I even suggested Ping increasing the memory pressure to "stress -m 250", it still succeeded to offline and remove. So I think this patch works to solve the issue. Thanks a lot for your help, all of you. High, will you post a formal patch in a separate thread? Meanwhile we found sometime onlining page may not add back all memory blocks on one memory board, then hot removing/adding them will cause kernel panic. I will investigate further and collect information, see if it's a kernel issue or udev issue. Thanks Baoquan > > > mean that this case cannot go back through the loop: but that's fine for > > the page migration case, and even if used more widely, is limited by the > > "Stop walking if it's locked" optimization in wake_page_function(). > > > > Add interface put_and_wait_on_page_locked() to do this, using negative > > value of the lock arg to wait_on_page_bit_common() to implement it. > > No interruptible or killable variant needed yet, but they might follow: > > I have a vague notion that reporting -EINTR should take precedence over > > return from wait_on_page_bit_common() without knowing the page state, > > so arrange it accordingly - but that may be nothing but pedantic. > > > > shrink_page_list()'s __ClearPageLocked(): that was a surprise! this > > survived a lot of testing before that showed up. It does raise the > > question: should is_page_cache_freeable() and __remove_mapping() now > > treat a PG_waiters page as if an extra reference were held? Perhaps, > > but I don't think it matters much, since shrink_page_list() already > > had to win its trylock_page(), so waiters are not very common there: I > > noticed no difference when trying the bigger change, and it's surely not > > needed while put_and_wait_on_page_locked() is only for page migration. > > > > Signed-off-by: Hugh Dickins > > --- > > ... > > > @@ -1100,6 +1111,17 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q, > > ret = -EINTR; > > break; > > } > > + > > + if (lock < 0) { > > + /* > > + * We can no longer safely access page->flags: > > Hmm... > > > + * even if CONFIG_MEMORY_HOTREMOVE is not enabled, > > + * there is a risk of waiting forever on a page reused > > + * for something that keeps it locked indefinitely. > > + * But best check for -EINTR above before breaking. > > + */ > > + break; > > + } > > } > > > > finish_wait(q, wait); > > ... the code continues by: > > if (thrashing) { > if (!PageSwapBacked(page)) > > So maybe we should not set 'thrashing' true when lock < 0? > > Thanks! > Vlastimil