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

[Pekka Enberg]

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?

			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) |


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