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[66.187.233.206]) by smtp.gmail.com with ESMTPSA id a11sm3048644qkc.123.2019.10.10.10.48.06 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 10 Oct 2019 10:48:07 -0700 (PDT) Message-ID: <1570729686.5937.30.camel@lca.pw> Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() From: Qian Cai To: Michal Hocko Cc: Petr Mladek , Christian Borntraeger , Heiko Carstens , sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org, john.ogness@linutronix.de, akpm@linux-foundation.org, Vasily Gorbik , Peter Oberparleiter , david@redhat.com, linux-kernel@vger.kernel.org Date: Thu, 10 Oct 2019 13:48:06 -0400 In-Reply-To: <20191010173040.GK18412@dhcp22.suse.cz> References: <20191009162339.GI6681@dhcp22.suse.cz> <6AAB77B5-092B-43E3-9F4B-0385DE1890D9@lca.pw> <20191010105927.GG18412@dhcp22.suse.cz> <1570713112.5937.26.camel@lca.pw> <20191010141820.GI18412@dhcp22.suse.cz> <1570718858.5937.28.camel@lca.pw> <20191010173040.GK18412@dhcp22.suse.cz> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.6 (3.22.6-10.el7) Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2019-10-10 at 19:30 +0200, Michal Hocko wrote: > On Thu 10-10-19 10:47:38, Qian Cai wrote: > > On Thu, 2019-10-10 at 16:18 +0200, Michal Hocko wrote: > > > On Thu 10-10-19 09:11:52, Qian Cai wrote: > > > > On Thu, 2019-10-10 at 12:59 +0200, Michal Hocko wrote: > > > > > On Thu 10-10-19 05:01:44, Qian Cai wrote: > > > > > > > > > > > > > > > > > > > On Oct 9, 2019, at 12:23 PM, Michal Hocko wrote: > > > > > > > > > > > > > > If this was only about the memory offline code then I would agree. But > > > > > > > we are talking about any printk from the zone->lock context and that is > > > > > > > a bigger deal. Besides that it is quite natural that the printk code > > > > > > > should be more universal and allow to be also called from the MM > > > > > > > contexts as much as possible. If there is any really strong reason this > > > > > > > is not possible then it should be documented at least. > > > > > > > > > > > > Where is the best place to document this? I am thinking about under > > > > > > the “struct zone” definition’s lock field in mmzone.h. > > > > > > > > > > I am not sure TBH and I do not think we have reached the state where > > > > > this would be the only way forward. > > > > > > > > How about I revised the changelog to focus on memory offline rather than making > > > > a rule that nobody should call printk() with zone->lock held? > > > > > > If you are to remove the CONFIG_DEBUG_VM printk then I am all for it. I > > > am still not convinced that fiddling with dump_page in the isolation > > > code is justified though. > > > > No, dump_page() there has to be fixed together for memory offline to be useful. > > What's the other options it has here? > > I would really prefer to not repeat myself > http://lkml.kernel.org/r/20191010074049.GD18412@dhcp22.suse.cz Care to elaborate what does that mean? I am confused on if you finally agree on no printk() while held zone->lock or not. You said "If there is absolutely no way around that then we might have to bite a bullet and consider some of MM locks a land of no printk." which makes me think you agreed, but your stance from the last reply seems you were opposite to it. > > > By not holding zone->lock in dump_page() > > from set_migratetype_isolate(), it even has a good side-effect to increase the > > system throughput as dump_page() could be time-consuming. It may make the code a > > bit cleaner by introducing a has_unmovable_pages_locked() version. > > I do not see why we should really optimize this cold path.