Re: [PATCH] fix double unlock_page() in 2.6.26-rc5-mm3 kernel BUGat mm/filemap.c:575!

[Lee Schermerhorn]

On Fri, 2008-06-13 at 11:30 -0400, Lee Schermerhorn wrote:
> On Thu, 2008-06-12 at 19:13 -0700, Andrew Morton wrote:
> > On Fri, 13 Jun 2008 10:44:44 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > 
> > > This is reproducer of panic. "quick fix" is attached.
> > 
> > Thanks - I put that in
> > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.26-rc5/2.6.26-rc5-mm3/hot-fixes/
> > 
> > > But I think putback_lru_page() should be re-designed.
> > 
> > Yes, it sounds that way.
> 
> Here's a proposed replacement patch that reworks putback_lru_page()
> slightly and cleans up the call sites.  I still want to balance the
> get_page() in isolate_lru_page() with a put_page() in putback_lru_page()
> for the primary users--vmscan and page migration.  So, I need to drop
> the lock before the put_page() when handed a page with null mapping and
> a single reference count as the page will be freed on put_page() and a
> locked page would bug out in free_pages_check()/bad_page().  
> 

Below is a fix to the "proposed replacement patch" posted on Friday.
Incorrect test for page->mapping().

Lee

Against:  2.6.26-rc5-mm3 

Incremental fix to my proposed patch to "fix double unlock_page() in
2.6.26-rc5-mm3 kernel BUG at mm/filemap.c:575".

"page_mapping(page)" should be "page->mapping" in VM_BUG_ON()s
introduced to m[un]lock_vma_page().

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@xxxxxx>

 mm/mlock.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Index: linux-2.6.26-rc5-mm3/mm/mlock.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/mm/mlock.c	2008-06-16 09:47:28.000000000 -0400
+++ linux-2.6.26-rc5-mm3/mm/mlock.c	2008-06-16 09:48:27.000000000 -0400
@@ -80,12 +80,12 @@ void __clear_page_mlock(struct page *pag
  * Mark page as mlocked if not already.
  * If page on LRU, isolate and putback to move to unevictable list.
  *
- * Called with page locked and page_mapping() != NULL.
+ * Called with page locked and page->mapping != NULL.
  */
 void mlock_vma_page(struct page *page)
 {
 	BUG_ON(!PageLocked(page));
-	VM_BUG_ON(!page_mapping(page));
+	VM_BUG_ON(!page->mapping);
 
 	if (!TestSetPageMlocked(page)) {
 		inc_zone_page_state(page, NR_MLOCK);
@@ -98,7 +98,7 @@ void mlock_vma_page(struct page *page)
 /*
  * called from munlock()/munmap() path with page supposedly on the LRU.
  *
- * Called with page locked and page_mapping() != NULL.
+ * Called with page locked and page->mapping != NULL.
  *
  * Note:  unlike mlock_vma_page(), we can't just clear the PageMlocked
  * [in try_to_munlock()] and then attempt to isolate the page.  We must
@@ -118,7 +118,7 @@ void mlock_vma_page(struct page *page)
 static void munlock_vma_page(struct page *page)
 {
 	BUG_ON(!PageLocked(page));
-	VM_BUG_ON(!page_mapping(page));
+	VM_BUG_ON(!page->mapping);
 
 	if (TestClearPageMlocked(page)) {
 		dec_zone_page_state(page, NR_MLOCK);


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