linux-kernel - Re: Regression - locking (all from 2.6.28)

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Date:	Mon, 16 Mar 2009 17:12:43 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	jan sonnek <ha2nny@...il.com>, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Whitcroft <apw@...dowen.org>, Pavel Machek <pavel@....cz>
Subject: Re: Regression - locking (all from 2.6.28)

Hi Dave,

> On Fri, 2009-03-06 at 18:00 +0000, Catalin Marinas wrote:
> > > I think you should be more worried about consistency rather than missing
> > > entries.  Take these two lines of code:
> > >
> > >       start_pfn = node->node_start_pfn;
> > >       /* hotplug occurs here */
> > >       end_pfn = start_pfn + node->node_spanned_pages;
> > >
> > > What if someone comes in and adds memory to the node, at the beginning
> > > of the node, after you have calculated start_pfn?  Try to think of what
> > > value you'll get for end_pfn and whether it is consistent and was *ever*
> > > valid at all.  Would that oops the kernel?
> >
> > I assume pfn_valid() should handle this and kmemleak wouldn't scan the
> > page, unless we need locks around pfn_valid as well but I haven't seen
> > any used in the kernel.
> 
> You assume incorrectly. :(
> 
> Take my above example, and assume that you have two nodes which are
> right next to each other.  You might run over the end of one node and
> into the next one.  Your pages will be pfn_valid() but you will be on
> the wrong node.

OK, thanks for taking the time to explain this. I currently added a
dependency on !MEMORY_HOTPLUG for kmemleak since holding the lock while
traversing the page structures is not really feasible.

> You could probably also use the memory hotplug mutex found here:
> 
> https://lists.linux-foundation.org/pipermail/linux-pm/2008-November/018884.html

That would be a better option for kmemleak as well.

-- 
Catalin

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