Re: [RFC] reduce hugetlb_instantiation_mutex usage

[Andrew Morton <akpm@osdl.org>]

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().

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