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Date:   Thu, 12 Sep 2019 13:11:50 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
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On Thu 12-09-19 13:24:25, Kirill A. Shutemov wrote:
> On Thu, Sep 12, 2019 at 11:19:25AM +0200, Michal Hocko wrote:
> > 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.
> 
> I don't necessary follow why shuffling is more integral to page allocator
> than reporting would be. It's next to shutffle.c under mm/ and integrated
> in a simillar way.

The main difference from my understanding is that the page reporting is
a more generic looking feature which might grow different users over
time yet there is a hardcoded set of restrictions to the allocator. Page
shuffling is an integral part of the allocator without any other
visibility outside.

-- 
Michal Hocko
SUSE Labs