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

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Date:   Fri, 04 Oct 2019 09:56:00 -0400
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
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, 2019-10-04 at 15:38 +0200, Michal Hocko wrote:
> 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.

Basically, printk() while holding the zone_lock will be problematic as console
is doing the opposite as it always needs to allocate some memory. Then, it will
always find some way to form this chain,

console_lock -> * -> zone_lock.

> 
> > 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.
>