linux-kernel - Re: [PATCH 3/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

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Date:   Tue, 11 Sep 2018 15:35:04 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        pavel.tatashin@...rosoft.com, Michal Hocko <mhocko@...e.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 3/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap

On Mon, Sep 10, 2018 at 4:43 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> From: Alexander Duyck <alexander.h.duyck@...el.com>
>
> The ZONE_DEVICE pages were being initialized in two locations. One was with
> the memory_hotplug lock held and another was outside of that lock. The
> problem with this is that it was nearly doubling the memory initialization
> time. Instead of doing this twice, once while holding a global lock and
> once without, I am opting to defer the initialization to the one outside of
> the lock. This allows us to avoid serializing the overhead for memory init
> and we can instead focus on per-node init times.
>
> One issue I encountered is that devm_memremap_pages and
> hmm_devmmem_pages_create were initializing only the pgmap field the same
> way. One wasn't initializing hmm_data, and the other was initializing it to
> a poison value. Since this is something that is exposed to the driver in
> the case of hmm I am opting for a third option and just initializing
> hmm_data to 0 since this is going to be exposed to unknown third party
> drivers.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---
>  include/linux/mm.h |    2 +
>  kernel/memremap.c  |   24 +++++---------
>  mm/hmm.c           |   12 ++++---
>  mm/page_alloc.c    |   89 +++++++++++++++++++++++++++++++++++++++++++++++++++-

Hmm, why mm/page_alloc.c and not kernel/memremap.c for this new
helper? I think that would address the kbuild reports and keeps all
the devm_memremap_pages / ZONE_DEVICE special casing centralized. I
also think it makes sense to move memremap.c to mm/ rather than
kernel/ especially since commit 5981690ddb8f "memremap: split
devm_memremap_pages() and memremap() infrastructure". Arguably, that
commit should have went ahead with the directory move.