linux-kernel - Re: [PATCH] mm/page_alloc: Add a reason for reserved pages in has_unmovable

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Message-ID: <20191004133814.GM9578@dhcp22.suse.cz>
Date:   Fri, 4 Oct 2019 15:38:14 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: Add a reason for reserved pages in
 has_unmovable_pages()

On Fri 04-10-19 09:30:39, Qian Cai wrote:
> On Fri, 2019-10-04 at 15:07 +0200, Michal Hocko wrote:
> > On Fri 04-10-19 08:56:16, Qian Cai wrote:
> > [...]
> > > It might be a good time to rethink if it is really a good idea to dump_page()
> > > at all inside has_unmovable_pages(). As it is right now, it is a a potential
> > > deadlock between console vs memory offline. More details are in this thread,
> > > 
> > > https://lore.kernel.org/lkml/1568817579.5576.172.camel@lca.pw/
> > 
> > Huh. That would imply we cannot do any printk from that path, no?
> 
> Yes, or use something like printk_deferred()

This is just insane. The hotplug code is in no way special wrt printk.
It is never called from the printk code AFAIK and thus there is no real
reason why this particular code should be any special. Not to mention
it calls printk indirectly from a code that is shared with other code
paths.

> or it needs to rework of the current console locking which I have no
> clue yet.

Yes, if the lockdep is really referring to a real deadlock which I
haven't really explored.

-- 
Michal Hocko
SUSE Labs