Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 4 Oct 2019 16:41:50 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@suse.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Pavel Tatashin <pavel.tatashin@microsoft.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc: Add a reason for reserved pages in
 has_unmovable_pages()
Message-ID: <20191004144150.GO9578@dhcp22.suse.cz>
References: <1570090257-25001-1-git-send-email-anshuman.khandual@arm.com>
 <20191004105824.GD9578@dhcp22.suse.cz>
 <91128b73-9a47-100b-d3de-e83f0b941e9f@arm.com>
 <1570193776.5576.270.camel@lca.pw>
 <20191004130713.GK9578@dhcp22.suse.cz>
 <1570195839.5576.273.camel@lca.pw>
 <20191004133814.GM9578@dhcp22.suse.cz>
 <1570197360.5576.275.camel@lca.pw>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1570197360.5576.275.camel@lca.pw>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri 04-10-19 09:56:00, Qian Cai wrote:
> 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.

So this is not as much a hotplug specific problem but zone->lock ->
printk -> alloc chain that is a problem, right? Who is doing an
allocation from this atomic context? I do not see any atomic allocation
in kernel/printk/printk.c.
-- 
Michal Hocko
SUSE Labs