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

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Message-ID: <CAPcyv4goFsZR=O-P_KGrjV8hz0H=nqgYPGSGGLEnmtcagoTYrA@mail.gmail.com>
Date:   Mon, 29 Oct 2018 10:34:22 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     alexander.h.duyck@...ux.intel.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>,
        Zhang Yi <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 Mon, Oct 29, 2018 at 10:24 AM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Mon 29-10-18 10:01:28, Alexander Duyck wrote:
> > On Mon, 2018-10-29 at 17:35 +0100, Michal Hocko wrote:
[..]
> > > You are already doing per-page initialization so I fail to see a larger
> > > unit to operate on.
> >
> > I have a patch that makes it so that we can work at a pageblock level
> > since all of the variables with the exception of only the LRU and page
> > address fields can be precomputed. Doing that is one of the ways I was
> > able to reduce page init to 1/3 to 1/4 of the time it was taking
> > otherwise in the case of deferred page init.
>
> You still have to call set_page_links for each page. But let's assume we
> can do initialization per larger units. Nothing really prevent to hide
> that into constructor as well.

A constructor / indirect function call makes sense when there are
multiple sub-classes of object initialization, on the table I only see
3 cases: typical hotplug, base ZONE_DEVICE, ZONE_DEVICE + HMM. I think
we can look to move the HMM special casing out of line, then we're
down to 2. Even at 3 cases we're better off open-coding than a
constructor for such a low number of sub-cases to handle. I do not
foresee more cases arriving, so I struggle to see what the constructor
buys us in terms of code readability / maintainability?