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Date:   Thu, 2 Apr 2020 09:34:10 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Shile Zhang <shile.zhang@linux.alibaba.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        David Hildenbrand <david@redhat.com>,
        James Morris <jmorris@namei.org>,
        Sasha Levin <sashal@kernel.org>
Subject: Re: [PATCH] mm: initialize deferred pages with interrupts enabled
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On Wed 01-04-20 16:27:33, Pavel Tatashin wrote:
> On Wed, Apr 1, 2020 at 3:57 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Wed 01-04-20 15:32:38, Pavel Tatashin wrote:
> > > Initializing struct pages is a long task and keeping interrupts disabled
> > > for the duration of this operation introduces a number of problems.
> > >
> > > 1. jiffies are not updated for long period of time, and thus incorrect time
> > >    is reported. See proposed solution and discussion here:
> > >    lkml/20200311123848.118638-1-shile.zhang@linux.alibaba.com
> >
> > http://lkml.kernel.org/r/20200311123848.118638-1-shile.zhang@linux.alibaba.com
> >
> > > 2. It prevents farther improving deferred page initialization by allowing
> > >    inter-node multi-threading.
> > >
> > > We are keeping interrupts disabled to solve a rather theoretical problem
> > > that was never observed in real world (See 3a2d7fa8a3d5).
> > >
> > > Lets keep interrupts enabled. In case we ever encounter a scenario where
> > > an interrupt thread wants to allocate large amount of memory this early in
> > > boot we can deal with that by growing zone (see deferred_grow_zone()) by
> > > the needed amount before starting deferred_init_memmap() threads.
> > >
> > > Before:
> > > [    1.232459] node 0 initialised, 12058412 pages in 1ms
> > >
> > > After:
> > > [    1.632580] node 0 initialised, 12051227 pages in 436ms
> > >
> >
> > Fixes: 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred pages")
> > > Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> >
> > I would much rather see pgdat_resize_lock completely out of both the
> > allocator and deferred init path altogether but this can be done in a
> > separate patch. This one looks slightly safer for stable backports.
> 
> This is what I wanted to do, but after studying deferred_grow_zone(),
> I do not see a simple way to solve this. It is one thing to fail an
> allocation, and it is another thing to have a corruption because of
> race.

Let's discuss deferred_grow_zone after this all settles down. I still
have to study it because I wasn't aware that this is actually a page
allocator path relying on the resize lock. My recollection was that the
resize lock is only about memory hotplug. Your patches flew by and I
didn't have time to review them back then. So I have to admit I have
seen the resize lock too simple.
-- 
Michal Hocko
SUSE Labs