Re: [RFC] reduce hugetlb_instantiation_mutex usage

['David Gibson' <david@gibson.dropbear.id.au>]

On Mon, Oct 30, 2006 at 06:54:46PM -0800, Chen, Kenneth W wrote:
> David Gibson wrote on Thursday, October 26, 2006 9:06 PM
> > > Alternatively, we could put the page into pagecache whether or not the
> > > mapping is MAP_SHARED.  Then pull it out again prior to unlocking it if
> > > it's MAP_PRIVATE.  So we're using pagecache just as a way for the
> > > concurrent faulter to locate the page.
> > 
> > Hrm.. interesting if we can make it work.  I'd be worried about cases
> > with concurrent PRIVATE and SHARED pages on the same file offset.
> 
> I got side tracked on to the radix-tree stuff.  The comments in
> hugetlb_no_page() make me wonder whether we have a race issue on
> private mapping:
> 
>         /*
>          * Use page lock to guard against racing truncation
>          * before we get page_table_lock.
>          */
> 
> Private mapping won't use radix tree during instantiation.  What protects
> racy truncate against fault in that scenario?  Don't we have a bug here?

Not at present, because the hugetlb_instantiation_mutex protects both
fault paths.  But with Andrew's patch as it stands, yes.  As I said in
a previous email.  The libhugetlbfs testsuite now has a testcase for
the MAP_PRIVATE as well as the MAP_SHARED version of the race.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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