Messages in this thread Patch in this message |  | | | Date | Sat, 15 Oct 2005 23:35:50 +1000 | | From | Nick Piggin <> | | Subject | Re: Possible memory ordering bug in page reclaim? | |
Herbert Xu wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> 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@suse.de>
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;
} |  |