Re: [RFC] reduce hugetlb_instantiation_mutex usage

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

On Thu, Oct 26, 2006 at 03:44:51PM -0700, Andrew Morton wrote:
> On Thu, 26 Oct 2006 15:17:20 -0700
> "Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:
> 
> > First rev of patch to allow hugetlb page fault to scale.
> > 
> > hugetlb_instantiation_mutex was introduced to prevent spurious allocation
> > failure in a corner case: two threads race to instantiate same page with
> > only one free page left in the global pool.  However, this global
> > serialization hurts fault performance badly as noted by Christoph Lameter.
> > This patch attempt to cut back the use of mutex only when free page resource
> > is limited, thus allow fault to scale in most common cases.
> >
> 
> ug.
> 
> How about we kill that instantiation_mutex thing altogether and fix
> the original bug in a better fashion?  Like...
> 
> In hugetlb_no_page():
> 
> retry:
> 	page = find_lock_page(...)
> 	if (!page) {
> 		write_lock_irq(&mapping->tree_lock);
> 		if (radix_tree_lookup(...)) {
> 			write_unlock_irq(tree_lock);
> 			goto retry;
> 		}
> 		page = alloc_huge_page(...);
> 		if (!page)
> 			bail;
> 		radix_tree_insert(...);
> 		SetPageLocked(page);
> 		write_unlock_irq(tree_lock);
> 		clear_huge_page(...);
> 	}
> 
> 	<stick it in page tables>
> 
> 	unlock_page(page);
> 
> The key points:
> 
> - Use tree_lock to prevent the race
> 
> - allocate the hugepage inside tree_lock so we never get into this
>   two-threads-tried-to-allocate-the-final-page problem.
> 
> - The hugepage is zeroed without locks held, under lock_page()
> 
> - lock_page() is used to make the other thread(s) sleep while the winner
>   thread is zeroing out the page.
> 
> It means that rather a lot of add_to_page_cache() will need to be copied
> into hugetlb_no_page().

This handles the case of processes racing on a shared mapping, but not
the case of threads racing on a private mapping.  In the latter case
the race ends at the set_pte() rather than the add_to_page_cache()
(well, strictly with the whole page_table_lock atomic lump).  And we
can't move the clear after the set_pte() :(.

-- 
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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