Re: BUG: Bad page state [was: Strange oopses in 2.6.30]

[Mel Gorman <mel@csn.ul.ie>]

On Mon, Jun 22, 2009 at 10:42:09AM +0300, Pekka Enberg wrote:
> On Mon, 2009-06-22 at 11:39 +0900, KOSAKI Motohiro wrote:
> > (cc to Mel and some reviewer)
> > 
> > > Flags are:
> > > 0000000000400000 -- __PG_MLOCKED
> > > 800000000050000c -- my page flags
> > >         3650000c -- Maxim's page flags
> > > 0000000000693ce1 -- my PAGE_FLAGS_CHECK_AT_FREE
> > 
> > I guess commit da456f14d (page allocator: do not disable interrupts in
> > free_page_mlock()) is a bit wrong.
> > 
> > current code is:
> > -------------------------------------------------------------
> > static void free_hot_cold_page(struct page *page, int cold)
> > {
> > (snip)
> >         int clearMlocked = PageMlocked(page);
> > (snip)
> >         if (free_pages_check(page))
> >                 return;
> > (snip)
> >         local_irq_save(flags);
> >         if (unlikely(clearMlocked))
> >                 free_page_mlock(page);
> > -------------------------------------------------------------
> > 
> > Oh well, we remove PG_Mlocked *after* free_pages_check().
> > Then, it makes false-positive warning.
> > 
> > Sorry, my review was also wrong. I think reverting this patch is better ;)
> 
> Well, I am not sure we need to revert the patch. I'd argue it's simply a
> bug in free_pages_check() that can be fixed with something like this.
> Mel, what do you think?
> 

I think you removed the check for the wrong flag - PG_locked vs
PG_mlocked :).

That aside, I reckon your intention was not far off the mark. I posted a
separate patch to see about warning once when PG_mlocked is set and
counting the event. When the warning appears, it's not world ending but
chances are it's something that needs to be fixed up.

> 			Pekka
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index d6792f8..b002b65 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -385,7 +385,7 @@ static inline void __ClearPageTail(struct page *page)
>   * these flags set.  It they are, there is a problem.
>   */
>  #define PAGE_FLAGS_CHECK_AT_FREE \
> -	(1 << PG_lru	 | 1 << PG_locked    | \
> +	(1 << PG_lru	 | \
>  	 1 << PG_private | 1 << PG_private_2 | \
>  	 1 << PG_buddy	 | 1 << PG_writeback | 1 << PG_reserved | \
>  	 1 << PG_slab	 | 1 << PG_swapcache | 1 << PG_active | \
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a5f3c27..ff7c713 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -497,6 +497,11 @@ static void free_page_mlock(struct page *page) { }
>  
>  static inline int free_pages_check(struct page *page)
>  {
> +	/*
> +	 * Note: the page can have PG_mlock set here because we clear it
> +	 * lazily to avoid unnecessary disabling and enabling of interrupts in
> +	 * page free fastpath.
> +	 */
>  	if (unlikely(page_mapcount(page) |
>  		(page->mapping != NULL)  |
>  		(atomic_read(&page->_count) != 0) |
> 
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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