Re: Regression - locking (all from 2.6.28)

[Catalin Marinas <catalin.marinas@arm.com>]

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