Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Message-ID: <cca53aa628a25ead13a2f71060b56bde66e19d05.camel@linux.intel.com>
Subject: Re: [PATCH v9 3/8] mm: Move set/get_pcppage_migratetype to mmzone.h
From:   Alexander Duyck <alexander.h.duyck@linux.intel.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     virtio-dev@lists.oasis-open.org, kvm@vger.kernel.org,
        mst@redhat.com, catalin.marinas@arm.com, david@redhat.com,
        dave.hansen@intel.com, linux-kernel@vger.kernel.org,
        willy@infradead.org, mhocko@kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, will@kernel.org,
        linux-arm-kernel@lists.infradead.org, osalvador@suse.de,
        yang.zhang.wz@gmail.com, pagupta@redhat.com,
        konrad.wilk@oracle.com, nitesh@redhat.com, riel@surriel.com,
        lcapitulino@redhat.com, wei.w.wang@intel.com, aarcange@redhat.com,
        ying.huang@intel.com, pbonzini@redhat.com,
        dan.j.williams@intel.com, fengguang.wu@intel.com,
        kirill.shutemov@linux.intel.com
Date:   Mon, 09 Sep 2019 11:01:04 -0700
In-Reply-To: <20190909095608.jwachx3womhqmjbl@box>
References: <20190907172225.10910.34302.stgit@localhost.localdomain>
         <20190907172528.10910.37051.stgit@localhost.localdomain>
         <20190909095608.jwachx3womhqmjbl@box>
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.30.5 (3.30.5-1.fc29) 
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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