linux-kernel - Re: [PATCH] mm: get_nid_for

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Message-Id: <20090227134616.982fb73a.akpm@linux-foundation.org>
Date:	Fri, 27 Feb 2009 13:46:16 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Gary Hade <garyhade@...ibm.com>
Cc:	roel.kluin@...il.com, garyhade@...ibm.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	y-goto@...fujitsu.com
Subject: Re: [PATCH] mm: get_nid_for_pfn() returns int

On Fri, 27 Feb 2009 13:33:40 -0800
Gary Hade <garyhade@...ibm.com> wrote:

> On Fri, Feb 27, 2009 at 03:56:40PM +0100, roel kluin wrote:
> > >> > > get_nid_for_pfn() returns int
> > 
> > >> > My mistake. __Good catch.
> > 
> > >> Presumably the (nid < 0) case has never happened.
> > >
> > > We do know that it is happening on one system while creating
> > > a symlink for a memory section so it should also happen on
> > > the same system if unregister_mem_sect_under_nodes() were
> > > called to remove the same symlink.
> > >
> > > The test was actually added in response to a problem with an
> > > earlier version reported by Yasunori Goto where one or more
> > > of the leading pages of a memory section on the 2nd node of
> > > one of his systems was uninitialized because I believe they
> > > coincided with a memory hole. __The earlier version did not
> > > ignore uninitialized pages and determined the nid by considering
> > > only the 1st page of each memory section. __This caused the
> > > symlink to the 1st memory section on the 2nd node to be
> > > incorrectly created in /sys/devices/system/node/node0 instead
> > > of /sys/devices/system/node/node1. __The problem was fixed by
> > > adding the test to skip over uninitialized pages.
> > >
> > > I suspect we have not seen any reports of the non-removal
> > > of a symlink due to the incorrect declaration of the nid
> > > variable in unregister_mem_sect_under_nodes() because
> > > __- systems where a memory section could have an uninitialized
> > > __ __range of leading pages are probably rare.
> > > __- memory remove is probably not done very frequently on the
> > > __ __systems that are capable of demonstrating the problem.
> > > __- lingering symlink(s) that should have been removed may
> > > __ __have simply gone unnoticed.
> > >>
> > >> Should we retain the test?
> > >
> > > Yes.
> > >
> > >>
> > >> Is silently skipping the node in that case desirable behaviour?
> > >
> > > It actually silently skips pages (not nodes) in it's quest
> > > for valid nids for all the nodes that the memory section scans.
> > > This is definitely desirable.
> > >
> > > I hope this answers your questions.
> > 
> > This still isn't applied, was it lost?
> 
> It is still lingering in -mm:
> http://userweb.kernel.org/~akpm/mmotm/broken-out/mm-get_nid_for_pfn-returns-int.patch
> 

Should it unlinger?  I have it in the 2.6.30 pile.  Does it actually
fix a demonstrable bug?  
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