From: "Chen, Kenneth W" <kenneth.w.chen@intel.com>
To: "'David Gibson'" <david@gibson.dropbear.id.au>,
       "Andrew Morton" <akpm@osdl.org>
Cc: <linux-kernel@vger.kernel.org>
Subject: RE: Hugepage regression
Date: Tue, 10 Oct 2006 10:35:50 -0700
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David Gibson wrote on Tuesday, October 10, 2006 2:16 AM
> > > It seems commit fe1668ae5bf0145014c71797febd9ad5670d5d05 causes a
> > > hugepage regression.  A git bisect points the finger at that commit
> > > for causing an oops in the 'alloc-instantiate-race' test from the
> > > libhugetlbfs testsuite.
> > > 
> > > Still looking to determine the reason it breaks things.
> > > 
> > 
> > It's assuming that unmap_hugepage_range() is always freeing these pages. 
> > If the page is shared by another mapping, bad things will happen: the
> > threads fight over page->lru.
> > 
> > Doing
> > 
> > +	if (page_count(page) == 1)
> > 		list_add(&page->lru, &page_list);
> > 
> > might help.  But then we miss the tlb flush in rare racy conditions.
> 
> Well, there'd need to be an else doing a put_page(), too.
> 
> Looks like the fundamental problem is that a list is not a suitable
> data structure for gathering here, since it's not truly local.  We
> should probably change it to a small array, like in the normal tlb
> gather structure.  If we run out of space we can force the tlb flush
> and keep going.


With the pending shared page table for hugetlb currently sitting in -mm,
we serialize the all hugetlb unmap with a per file i_mmap_lock.  This
race could well be solved by that pending patch?

http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.19-rc1/2.6.19-rc1-mm1/broken-out/shared-page-table-for-hugetlb-page-v
4.patch

- Ken
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