linux-kernel - Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

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Date:   Thu, 10 Oct 2019 19:30:40 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     Petr Mladek <pmladek@...e.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        sergey.senozhatsky.work@...il.com, rostedt@...dmis.org,
        peterz@...radead.org, linux-mm@...ck.org,
        john.ogness@...utronix.de, akpm@...ux-foundation.org,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>, david@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

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 <mhocko@...nel.org> 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

> 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.
-- 
Michal Hocko
SUSE Labs