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Date:   Sat, 5 Oct 2019 14:22:32 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Qian Cai <cai@lca.pw>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-mm@kvack.org, Vlastimil Babka <vbabka@suse.cz>,
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        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()
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On Fri, 4 Oct 2019 16:41:50 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> > > 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.

Apparently some console drivers can do memory allocation on the printk()
path.

This behavior is daft, IMO.  Have we identified which ones and looked
into fixing them?