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

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Message-ID: <20090227213340.GB7174@us.ibm.com>
Date:	Fri, 27 Feb 2009 13:33:40 -0800
From:	Gary Hade <garyhade@...ibm.com>
To:	roel kluin <roel.kluin@...il.com>
Cc:	Gary Hade <garyhade@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	lkml <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, 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

Gary

-- 
Gary Hade
System x Enablement
IBM Linux Technology Center
503-578-4503  IBM T/L: 775-4503
garyhade@...ibm.com
http://www.ibm.com/linux/ltc

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