From: KY Srinivasan <kys@microsoft.com>
To: Dave Hansen <dave@sr71.net>
CC: Michal Hocko <mhocko@suse.cz>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
        "andi@firstfloor.org" <andi@firstfloor.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "kamezawa.hiroyuki@gmail.com" <kamezawa.hiroyuki@gmail.com>,
        "hannes@cmpxchg.org" <hannes@cmpxchg.org>,
        "yinghan@google.com" <yinghan@google.com>,
        "jasowang@redhat.com" <jasowang@redhat.com>,
        "kay@vrfy.org" <kay@vrfy.org>
Subject: RE: [PATCH 1/1] Drivers: base: memory: Export symbols for onlining
 memory blocks
Thread-Topic: [PATCH 1/1] Drivers: base: memory: Export symbols for onlining
 memory blocks
Thread-Index: AQHOhLBUexA6H+ozR02R0nup8ZuRHJlwpwQAgAGspICAABWHgIAABmSQgAACj4CAAA/eQIABjleAgAArP0CAAB1DAIAAtuGAgAAyoQCAAESVgIAACFxw
Date: Thu, 25 Jul 2013 15:49:00 +0000
Message-ID: <828d748273884d6f9c24b658964f97c3@SN2PR03MB061.namprd03.prod.outlook.com>
References: <1374261785-1615-1-git-send-email-kys@microsoft.com>
 <20130722123716.GB24400@dhcp22.suse.cz>
 <e06fced3ca42408b980f8aa68f4a29f3@SN2PR03MB061.namprd03.prod.outlook.com>
 <51EEA11D.4030007@intel.com>
 <3318be0a96cb4d05838d76dc9d088cc0@SN2PR03MB061.namprd03.prod.outlook.com>
 <51EEA89F.9070309@intel.com>
 <9f351a549e76483d9148f87535567ea0@SN2PR03MB061.namprd03.prod.outlook.com>
 <51F00415.8070104@sr71.net>
 <d1f80c05986b439cbeef12bcd595b264@BLUPR03MB050.namprd03.prod.outlook.com>
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> -----Original Message-----
> From: Dave Hansen [mailto:dave@sr71.net]
> Sent: Thursday, July 25, 2013 11:04 AM
> To: KY Srinivasan
> Cc: Michal Hocko; gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> andi@firstfloor.org; akpm@linux-foundation.org; linux-mm@kvack.org;
> kamezawa.hiroyuki@gmail.com; hannes@cmpxchg.org; yinghan@google.com;
> jasowang@redhat.com; kay@vrfy.org
> Subject: Re: [PATCH 1/1] Drivers: base: memory: Export symbols for onlining
> memory blocks
> 
> On 07/25/2013 04:14 AM, KY Srinivasan wrote:
> > As promised, I have sent out the patches for (a) an implementation of an in-
> kernel API
> > for onlining  and a consumer for this API. While I don't know the exact reason
> why the
> > user mode code is delayed (under some low memory conditions), what is the
> harm in having
> > a mechanism to online memory that has been hot added without involving user
> space code.
> 
> KY, your potential problem, not being able to online more memory because
> of a shortage of memory, is a serious one.

All I can say is that the online is not happening within the allowed time (5 seconds in
the current code).
> 
> However, this potential issue exists in long-standing code, and
> potentially affects all users of memory hotplug.  The problem has not
> been described in sufficient detail for the rest of the developers to
> tell if you are facing a new problem, or whether *any* proposed solution
> will help the problem you face.
> 
> Your propsed solution changes the semantics of existing user/kernel
> interfaces, duplicates existing functionality, and adds code complexity
> to the kernel.

How so? All I am doing is to use the existing infrastructure to online
memory. The only change I have made is to export an API that allows
onlining without involving any user space code. I don't see how  this
adds complexity to the kernel. This would be an useful extension as can
be seen from its usage in the Hyper-V balloon driver.

In my particular use case, I need to wait for the memory to be onlined before
I can proceed to hot add the next chunk. This synchronization can be completely
avoided if we can avoid the involvement of user level code. I would submit to you that
this is a valid use case that we ought to be able to support.

Regards,

K. Y


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