Re: WTH is going on with memory hotplug sysf interface

From: Michal Hocko <mhocko@kernel.org>
To: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>
Cc: Igor Mammedov <imammedo@redhat.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	David Rientjes <rientjes@google.com>,
	Daniel Kiper <daniel.kiper@oracle.com>,
	linux-api@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	linux-s390@vger.kernel.org, xen-devel@lists.xenproject.org,
	linux-acpi@vger.kernel.org, qiuxishi@huawei.com,
	toshi.kani@hpe.com, xieyisheng1@huawei.com, slaoub@gmail.com,
	iamjoonsoo.kim@lge.com, vbabka@suse.cz,
	Reza Arbab <arbab@linux.vnet.ibm.com>
Subject: Re: WTH is going on with memory hotplug sysf interface
Date: Tue, 14 Mar 2017 17:20:03 +0100	[thread overview]
Message-ID: <20170314162003.GD7772@dhcp22.suse.cz> (raw)
In-Reply-To: <99f14975-f89f-4484-6ae1-296b242d4bf9@gmail.com>

On Tue 14-03-17 12:05:59, YASUAKI ISHIMATSU wrote:
> 
> 
> On 03/13/2017 05:19 AM, Michal Hocko wrote:
> >On Fri 10-03-17 12:39:27, Yasuaki Ishimatsu wrote:
> >>On 03/10/2017 08:58 AM, Michal Hocko wrote:
[...]
> >>># echo online_movable > /sys/devices/system/memory/memory34/state
> >>># grep . /sys/devices/system/memory/memory3?/valid_zones
> >>>/sys/devices/system/memory/memory32/valid_zones:Normal
> >>>/sys/devices/system/memory/memory33/valid_zones:Normal Movable
> >>>/sys/devices/system/memory/memory34/valid_zones:Movable Normal
> >>>
> >>
> >>I think there is no strong reason which kernel has the restriction.
> >>By setting the restrictions, it seems to have made management of
> >>these zone structs simple.
> >
> >Could you be more specific please? How could this make management any
> >easier when udev is basically racing with the physical hotplug and the
> >result is basically undefined?
> >
> 
> When changing zone from NORMAL(N) to MOVALBE(M), we must resize both zones,
> zone->zone_start_pfn and zone->spanned_pages. Currently there is the
> restriction.
> 
> So we just simply change:
>   zone(N)->spanned_pages -= nr_pages
>   zone(M)->zone_start_pfn -= nr_pages

Yes I understand how this made the implementation simpler. I was
questioning how this made user management any easier. Changing
valid zones which races with the hotplug consumer (e.g. udev) sounds
like a terrible idea to me.

Anyway, it seems that the initial assumption/restriction that all
pages have to start on the zone Normal is not really needed. I have a
preliminary patch which removes that and associates newly added pages
with a zone at the online time and it seems to be working reasonably
well. I have to iron out some corners before I post it.
-- 
Michal Hocko
SUSE Labs