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;
Date:   Fri, 28 Feb 2020 15:25:36 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, richardw.yang@linux.intel.com,
        david@redhat.com, osalvador@suse.de, dan.j.williams@intel.com,
        rppt@linux.ibm.com, robin.murphy@arm.com
Subject: Re: [PATCH v2 4/7] mm/sparse.c: only use subsection map in VMEMMAP
 case
Message-ID: <20200228072536.GK24216@MiWiFi-R3L-srv>
References: <20200220043316.19668-1-bhe@redhat.com>
 <20200220043316.19668-5-bhe@redhat.com>
 <20200225095713.GL22443@dhcp22.suse.cz>
 <20200226035336.GF24216@MiWiFi-R3L-srv>
 <20200226091018.GD3771@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200226091018.GD3771@dhcp22.suse.cz>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 02/26/20 at 10:10am, Michal Hocko wrote:
> On Wed 26-02-20 11:53:36, Baoquan He wrote:
> > On 02/25/20 at 10:57am, Michal Hocko wrote:
> > > On Thu 20-02-20 12:33:13, Baoquan He wrote:
> > > > Currently, subsection map is used when SPARSEMEM is enabled, including
> > > > VMEMMAP case and !VMEMMAP case. However, subsection hotplug is not
> > > > supported at all in SPARSEMEM|!VMEMMAP case, subsection map is unnecessary
> > > > and misleading. Let's adjust code to only allow subsection map being
> > > > used in VMEMMAP case.
> > > 
> > > This really needs more explanation I believe. What exactly happens if
> > > somebody tries to hotremove a part of the section with !VMEMMAP? I can
> > > see that clear_subsection_map returns 0 but that is not an error code.
> > > Besides that section_deactivate doesn't propagate the error upwards.
> > > /me stares into the code
> > > 
> > > OK, I can see it now. It is relying on check_pfn_span to use the proper
> > > subsection granularity. This really begs for a comment in the code
> > > somewhere.
> > 
> > Yes, check_pfn_span() guards it. People have no way to hot add/remove
> > on non-section aligned block with !VMEMMAP.
> > 
> > I have added extra comment to above section_activate() to note this,
> > please check patch 5/7. Let me see how to add words to reflect the
> > check_pfn_span() guard thing.
> 
> An explicit note about check_pfn_span gating the proper alignement and
> sizing sounds sufficient to me.

It's fine to me, I will adjust the description.