linux-kernel - Re: [PATCH v9 3/8] mm: Move set/get_pcppage

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Date:   Mon, 09 Sep 2019 11:01:04 -0700
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     virtio-dev@...ts.oasis-open.org, kvm@...r.kernel.org,
        mst@...hat.com, catalin.marinas@....com, david@...hat.com,
        dave.hansen@...el.com, linux-kernel@...r.kernel.org,
        willy@...radead.org, mhocko@...nel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, osalvador@...e.de,
        yang.zhang.wz@...il.com, pagupta@...hat.com,
        konrad.wilk@...cle.com, nitesh@...hat.com, riel@...riel.com,
        lcapitulino@...hat.com, wei.w.wang@...el.com, aarcange@...hat.com,
        ying.huang@...el.com, pbonzini@...hat.com,
        dan.j.williams@...el.com, fengguang.wu@...el.com,
        kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v9 3/8] mm: Move set/get_pcppage_migratetype to mmzone.h

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@...ux.intel.com>
> > 
> > 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@...el.com>
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> 
> 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.