Re: [PATCH v9 3/8] mm: Move set/get_pcppage_migratetype to mmzone.h

[Alexander Duyck]

On Mon, 2019-09-09 at 12:56 +0300, Kirill A. Shutemov wrote:
> On Sat, Sep 07, 2019 at 10:25:28AM -0700, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > 
> > In order to support page reporting it will be necessary to store and
> > retrieve the migratetype of a page. To enable that I am moving the set and
> > get operations for pcppage_migratetype into the mm/internal.h header so
> > that they can be used outside of the page_alloc.c file.
> > 
> > Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> 
> I'm not sure that it's great idea to export this functionality beyond
> mm/page_alloc.c without any additional safeguards. How would we avoid to
> messing with ->index when the page is not in the right state of its
> life-cycle. Can we add some VM_BUG_ON()s here?

I am not sure what we would need to check on though. There are essentially
2 cases where we are using this. The first is the percpu page lists so the
value is set either as a result of __rmqueue_smallest or
free_unref_page_prepare. The second one which hasn't been added yet is for
the Reported pages case which I add with patch 6.

When I use it for page reporting I am essentially using the Reported flag
to identify what pages in the buddy list will have this value set versus
those that may not. I didn't explicitly define it that way, but that is
how I am using it so that the value cannot be trusted unless the Reported
flag is set.