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