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 17:59:25 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <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 Tue, Sep 11, 2018 at 5:51 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Tue, Sep 11, 2018 at 3:35 PM Dan Williams <dan.j.williams@...el.com> wrote:
>>
>> 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.
>
> The issue ends up being the fact that I would then have to start
> exporting infrastructure such as __init_single_page from page_alloc. I
> have some follow-up patches I am working on that will generate some
> other shared functions that can be used by both memmap_init_zone and
> memmap_init_zone_device, as well as pulling in some of the code from
> the deferred memory init.

You wouldn't need to export it, just make it public to mm/ in
mm/internal.h, or a similar local header. With kernel/memremap.c moved
to mm/memremap.c this becomes even easier and better scoped for the
shared symbols.