Re: [patch] mm: fix assertion

[Christoph Lameter <cl@linux-foundation.org>]

You still have a

  VM_BUG_ON(PageTail(page));

in page_cache_get_speculative() which will verify that the
successfully acquired speculative reference is not a compound tail.

Reviewed-by: Christoph Lameter <cl@linux-foundation.org>

On Wed, 14 Jan 2009, Nick Piggin wrote:

> (I ran into this when debugging the lockless pagecache barrier problem btw)
>
> --
>
> This assertion is incorrect for lockless pagecache. By definition if we have an
> unpinned page that we are trying to take a speculative reference to, it may
> become the tail of a compound page at any time (if it is freed, then reallocated
> as a compound page).
>
> It was still a valid assertion for the vmscan.c LRU isolation case, but it
> doesn't seem incredibly helpful... if somebody wants it, they can put it back
> directly where it applies in the vmscan code.
>
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> ---
> Index: linux-2.6/include/linux/mm.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm.h	2009-01-05 14:53:29.000000000 +1100
> +++ linux-2.6/include/linux/mm.h	2009-01-05 14:53:54.000000000 +1100
> @@ -270,7 +270,6 @@ static inline int put_page_testzero(stru
>   */
>  static inline int get_page_unless_zero(struct page *page)
>  {
> -	VM_BUG_ON(PageTail(page));
>  	return atomic_inc_not_zero(&page->_count);
>  }
>
>
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