linux-kernel - RE: [PATCH 1/1] Drivers: base: memory: Export symbols for onlining memory blocks

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Date:	Thu, 25 Jul 2013 15:49:00 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Dave Hansen <dave@...1.net>
CC:	Michal Hocko <mhocko@...e.cz>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"kamezawa.hiroyuki@...il.com" <kamezawa.hiroyuki@...il.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"yinghan@...gle.com" <yinghan@...gle.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"kay@...y.org" <kay@...y.org>
Subject: RE: [PATCH 1/1] Drivers: base: memory: Export symbols for onlining
 memory blocks



> -----Original Message-----
> From: Dave Hansen [mailto:dave@...1.net]
> Sent: Thursday, July 25, 2013 11:04 AM
> To: KY Srinivasan
> Cc: Michal Hocko; gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> andi@...stfloor.org; akpm@...ux-foundation.org; linux-mm@...ck.org;
> kamezawa.hiroyuki@...il.com; hannes@...xchg.org; yinghan@...gle.com;
> jasowang@...hat.com; kay@...y.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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