From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id 15F576B2034 for ; Tue, 20 Nov 2018 08:38:27 -0500 (EST) Received: by mail-ed1-f70.google.com with SMTP id t2so1265163edb.22 for ; Tue, 20 Nov 2018 05:38:27 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id d14si3992230edq.144.2018.11.20.05.38.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 20 Nov 2018 05:38:25 -0800 (PST) Subject: Re: Memory hotplug softlock issue References: <20181115143204.GV23831@dhcp22.suse.cz> <20181116012433.GU2653@MiWiFi-R3L-srv> <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> From: Vlastimil Babka Message-ID: <3f1a82a8-f2aa-ac5e-e6a8-057256162321@suse.cz> Date: Tue, 20 Nov 2018 14:38:23 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins , Baoquan He Cc: Michal Hocko , David Hildenbrand , linux-mm@kvack.org, pifang@redhat.com, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, aarcange@redhat.com, Mel Gorman 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. > 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