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

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Date:   Thu, 10 Oct 2019 09:11:52 -0400
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
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, 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?