Re: [patch] mm: fix assertion

[Nick Piggin <npiggin@suse.de>]

On Thu, Jan 15, 2009 at 11:35:45AM -0600, Christoph Lameter wrote:
> 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.

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

Thanks

> 
> 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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