Re: 2.6.22 -mm merge plans -- vm bugfixes

[Hugh Dickins]

On Thu, 3 May 2007, Nick Piggin wrote:
> 
> The problem is that lock/unlock_page is expensive on powerpc, and
> if we improve that, we improve more than just the fault handler...
> 
> The attached patch gets performance up a bit by avoiding some
> barriers and some cachelines:

There's a strong whiff of raciness about this...
but I could very easily be wrong.

> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c	2007-05-02 15:00:26.000000000 +1000
> +++ linux-2.6/mm/filemap.c	2007-05-03 08:34:32.000000000 +1000
> @@ -532,11 +532,13 @@
>   */
>  void fastcall unlock_page(struct page *page)
>  {
> +	VM_BUG_ON(!PageLocked(page));
>  	smp_mb__before_clear_bit();
> -	if (!TestClearPageLocked(page))
> -		BUG();
> -	smp_mb__after_clear_bit(); 
> -	wake_up_page(page, PG_locked);
> +	ClearPageLocked(page);
> +	if (unlikely(test_bit(PG_waiters, &page->flags))) {
> +		clear_bit(PG_waiters, &page->flags);
> +		wake_up_page(page, PG_locked);
> +	}
>  }
>  EXPORT_SYMBOL(unlock_page);
>  
> @@ -568,6 +570,11 @@ __lock_page (diff -p would tell us!)
>  {
>  	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
>  
> +	set_bit(PG_waiters, &page->flags);
> +	if (unlikely(!TestSetPageLocked(page))) {

What happens if another cpu is coming through __lock_page at the
same time, did its set_bit, now finds PageLocked, and so proceeds
to the __wait_on_bit_lock?  But this cpu now clears PG_waiters,
so this task's unlock_page won't wake the other?

> +		clear_bit(PG_waiters, &page->flags);
> +		return;
> +	}
>  	__wait_on_bit_lock(page_waitqueue(page), &wait, sync_page,
>  							TASK_UNINTERRUPTIBLE);
>  }
-
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