From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id D1F686B26BD for ; Wed, 21 Nov 2018 12:31:27 -0500 (EST) Received: by mail-ed1-f72.google.com with SMTP id y35so3335476edb.5 for ; Wed, 21 Nov 2018 09:31:27 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id j7si1637936eda.326.2018.11.21.09.31.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 21 Nov 2018 09:31:25 -0800 (PST) Date: Wed, 21 Nov 2018 18:31:23 +0100 From: Michal Hocko Subject: Re: Memory hotplug softlock issue Message-ID: <20181121173123.GS12932@dhcp22.suse.cz> References: <20181116091409.GD14706@dhcp22.suse.cz> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins Cc: Baoquan He , Vlastimil Babka , David Hildenbrand , linux-mm@kvack.org, pifang@redhat.com, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, aarcange@redhat.com, Mel Gorman On Mon 19-11-18 21:44:41, 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. I would add the following for the "problem statement". Feel free to reuse per your preference: " An elevated reference count, however, stands in the way of migration and forces it to fail with a bad timing. This is especially a problem for memory offlining which retries for ever (or until the operation is terminated from userspace) because a heavy refault workload can trigger essentially an endless loop of migration failures. Therefore __migration_entry_wait is essentially harmful for the even it is waiting for. " > 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 > 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(). I would appreciate this would be more explicit about the existence of the elevated-ref-count problem but it reduces it to a tiny time window compared to the whole time the waiter is blocked. So a great improvement. > 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! and I can imagine a bad one. Do we really have to be so clever here? The unlock_page went away in the name of performance (a978d6f521063) and I would argue that this is a slow path where this is just not worth it. > 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 The patch looks good to me - quite ugly but it doesn't make the existing code much worse. With the problem described Vlastimil fixed, feel free to add Acked-by: Michal Hocko And thanks for a prompt patch. This is something I've been chasing for quite some time. __migration_entry_wait came to my radar only recently because this is an extremely volatile area. -- Michal Hocko SUSE Labs