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Date:   Thu, 12 Sep 2019 11:19:25 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Alexander Duyck <alexander.duyck@gmail.com>
Cc:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        virtio-dev@lists.oasis-open.org, kvm list <kvm@vger.kernel.org>,
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On Wed 11-09-19 08:12:03, Alexander Duyck wrote:
> On Wed, Sep 11, 2019 at 4:36 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Tue 10-09-19 14:23:40, Alexander Duyck wrote:
> > [...]
> > > We don't put any limitations on the allocator other then that it needs to
> > > clean up the metadata on allocation, and that it cannot allocate a page
> > > that is in the process of being reported since we pulled it from the
> > > free_list. If the page is a "Reported" page then it decrements the
> > > reported_pages count for the free_area and makes sure the page doesn't
> > > exist in the "Boundary" array pointer value, if it does it moves the
> > > "Boundary" since it is pulling the page.
> >
> > This is still a non-trivial limitation on the page allocation from an
> > external code IMHO. I cannot give any explicit reason why an ordering on
> > the free list might matter (well except for page shuffling which uses it
> > to make physical memory pattern allocation more random) but the
> > architecture seems hacky and dubious to be honest. It shoulds like the
> > whole interface has been developed around a very particular and single
> > purpose optimization.
> 
> How is this any different then the code that moves a page that will
> likely be merged to the tail though?

I guess you are referring to the page shuffling. If that is the case
then this is an integral part of the allocator for a reason and it is
very well obvious in the code including the consequences. I do not
really like an idea of hiding similar constrains behind a generic
looking feature which is completely detached from the allocator and so
any future change of the allocator might subtly break it.

> In our case the "Reported" page is likely going to be much more
> expensive to allocate and use then a standard page because it will be
> faulted back in. In such a case wouldn't it make sense for us to want
> to keep the pages that don't require faults ahead of those pages in
> the free_list so that they are more likely to be allocated?

OK, I was suspecting this would pop out. And this is exactly why I
didn't like an idea of an external code imposing a non obvious constrains
to the allocator. You simply cannot count with any ordering with the
page allocator. We used to distinguish cache hot/cold pages in the past
and pushed pages to the specific end of the free list but that has been
removed. There are other potential changes like that possible. Shuffling
is a good recent example.

Anyway I am not a maintainer of this code. I would really like to hear
opinions from Mel and Vlastimil here (now CCed - the thread starts
http://lkml.kernel.org/r/20190907172225.10910.34302.stgit@localhost.localdomain.
-- 
Michal Hocko
SUSE Labs