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

From: Michal Hocko
Date: Mon Oct 29 2018 - 13:45:54 EST

On Mon 29-10-18 10:34:22, Dan Williams wrote:
> On Mon, Oct 29, 2018 at 10:24 AM Michal Hocko <mhocko@xxxxxxxxxx> 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?

I haven't dreamed of ZONE_DEVICE and HMM few years back. But anyway,
let me note that I am not in love with callbacks. I find them to be a
useful abstraction. I can be convinced (by numbers) that special casing
inside the core hotplug code is really beneficial. But let's do that at
a single place.

All I am arguing against throughout this thread is the
memmap_init_zone_device and the whole code duplication just because zone
device need something special.

Michal Hocko