Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 24 Jun 2020 09:11:18 +0800
From:   Wei Yang <richard.weiyang@linux.alibaba.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Wei Yang <richard.weiyang@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section
Message-ID: <20200624011118.GA9422@L-31X9LVDL-1304.local>
Reply-To: Wei Yang <richard.weiyang@linux.alibaba.com>
References: <20200623094258.6705-1-richard.weiyang@linux.alibaba.com>
 <CAPcyv4ipnZ2jXd-obBk4KMGPNz4DMu0jGXFEEPCnST+A2zR+Uw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAPcyv4ipnZ2jXd-obBk4KMGPNz4DMu0jGXFEEPCnST+A2zR+Uw@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Jun 23, 2020 at 05:21:06PM -0700, Dan Williams wrote:
>On Tue, Jun 23, 2020 at 2:43 AM Wei Yang
><richard.weiyang@linux.alibaba.com> wrote:
>>
>> For early sections, we assumes its memmap will never be partially
>> removed. But current behavior breaks this.
>
>Where do we assume that?
>
>The primary use case for this was mapping pmem that collides with
>System-RAM in the same 128MB section. That collision will certainly be
>depopulated on-demand depending on the state of the pmem device. So,
>I'm not understanding the problem or the benefit of this change.

Hi, Dan

There is a discussion in the thread you just replied:

    mm/shuffle: don't move pages between zones and don't read garbage memmaps

Besides this, the comment in section_activate() says:

    * The early init code does not consider partially populated
    * initial sections, it simply assumes that memory will never be
    * referenced.  If we hot-add memory into such a section then we
    * do not need to populate the memmap and can simply reuse what
    * is already there.

Per my understanding, if we hot-add then hot-remove the sub-section, we may
not have a valid memmep for this part sub-section? Because we depopulate it at
hot-remove while we don't populate it when hot-add.

Is my understanding correct?

-- 
Wei Yang
Help you, Help me