Re: Possible memory ordering bug in page reclaim?

[Nick Piggin]

Herbert Xu wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > Well yes, that's on the store side (1, above). However can't a CPU
> > still speculatively (eg. guess the branch) load the page->flags
> > cacheline which might be satisfied from memory before the page->count
> > cacheline loads? Ie. you can still have the correct write ordering
> > but have incorrect read ordering?
> >
> > Because neither PageDirty nor page_count is a barrier, and there is
> > no read barrier between them.
>
>
> Yes you're right.  A read barrier is required here.
>
> I think Ben was actually agreeing with you.  He's just questioning
> whether the corresponding write barrier existed on CPU 1 (the answer
> to which is affirmative).
>  

Ah, that clears up my misunderstanding.

Yes I agree the write side is OK.

Thanks Ben and Herbert. I guess I should do a proper patch then.

--
SUSE Labs, Novell Inc.

In mm/vmscan.c, the page reclaim may have the following sequence 2
running concurrently with sequence 1 on another CPU:

1                                2
find_get_page();
write to page                    write_lock(tree_lock);
SetPageDirty();                  if (page_count != 2
put_page();                              || PageDirty())
                                     /* page dirty or busy */
				 else
				     /* free it */

The comment indicates that PageDirty must be checked *after* page_count
indicates there are no users of this page, which prevents the dirty bit
from being lost in the case that that sequence 2 might see the state of
PageDirty() *before* SetPageDirty() in 1, but page_count *after* put_page
in 1.

However, there is no read memory barrier there, and so nothing to stop a
CPU from loading page_count before PageDirty (ie. ->flags). Theoretically,
data corruption is possible.

Signed-off-by: Nick Piggin <npiggin@xxxxxxx>

Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c
+++ linux-2.6/mm/vmscan.c
@@ -511,7 +511,12 @@ static int shrink_list(struct list_head 
 		 * PageDirty _after_ making sure that the page is freeable and
 		 * not in use by anybody. 	(pagecache + us == 2)
 		 */
-		if (page_count(page) != 2 || PageDirty(page)) {
+		if (page_count(page) != 2) {
+			write_unlock_irq(&mapping->tree_lock);
+			goto keep_locked;
+		}
+		smp_rmb();
+		if (PageDirty(page)) {
 			write_unlock_irq(&mapping->tree_lock);
 			goto keep_locked;
 		}