linux-kernel - Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

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Date:   Wed, 10 Oct 2018 20:52:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Pasha Tatashin <pavel.tatashin@...rosoft.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        rppt@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        yi.z.zhang@...ux.intel.com
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap

On Wed 10-10-18 10:39:01, Alexander Duyck wrote:
> On 10/10/2018 10:24 AM, Michal Hocko wrote:
[...]
> > I thought I have already made it clear that these zone device hacks are
> > not acceptable to the generic hotplug code. If the current reserve bit
> > handling is not correct then give us a specific reason for that and we
> > can start thinking about the proper fix.
> 
> I might have misunderstood your earlier comment then. I thought you were
> saying that we shouldn't bother with setting the reserved bit. Now it sounds
> like you were thinking more along the lines of what I was here in my comment
> where I thought the bit should be cleared later in some code specifically
> related to DAX when it is exposing it for use to userspace or KVM.

It seems I managed to confuse myself completely. Sorry, it's been a long
day and I am sick so the brain doesn't work all that well. I will get
back to this tomorrow or on Friday with a fresh brain.

My recollection was that we do clear the reserved bit in
move_pfn_range_to_zone and we indeed do in __init_single_page. But then
we set the bit back right afterwards. This seems to be the case since
d0dc12e86b319 which reorganized the code. I have to study this some more
obviously.

-- 
Michal Hocko
SUSE Labs