Re: 2.6.18: Kernel BUG at mm/rmap.c:522
From: Nick Piggin
Date: Thu Oct 05 2006 - 02:29:36 EST
Dave Jones wrote:
On Wed, Oct 04, 2006 at 05:49:00PM +0200, Peter Zijlstra wrote:
> > > It is also nice if we can work out where the page actually came from. The
> > > following attached patch should help out a bit with that, if you could
> > > run with it?
> > Okay. I'll reboot with your patch and let you know if it crashes again.
> enable CONFIG_DEBUG_VM to get that.
Given this warnings still pops up from time to time, I question whether
putting that check under DEBUG_VM was such a good idea. It's not as
if it's a major performance impact. This has potential for us to lose
valuable debugging info for a few nanoseconds performance increase in
an already costly path.
Frustratingly, it usually doesn't tell us much (without my previous
patch), because it is normally some driver that has stuffed up their
refcounting and the page-> fields don't tell us where the page has
come from.
But..
This patch brings it back unconditionally, and moves the BUG()
into the if arm.
... this shouldn't hurt if gcc moves the unlikely code out of the
linear instruction stream, which I think it usually does. It shouldn't
cost _anything_ because we're already doing the branch for the BUG_ON
which you remove.
Signed-off-by: Dave Jones <davej@xxxxxxxxxx>
Thanks,
Acked-by: Nick Piggin <npiggin@xxxxxxx>
--- local-git/mm/rmap.c~ 2006-10-04 16:38:06.000000000 -0400
+++ local-git/mm/rmap.c 2006-10-04 16:38:24.000000000 -0400
@@ -576,15 +576,14 @@ void page_add_file_rmap(struct page *pag
void page_remove_rmap(struct page *page)
{
if (atomic_add_negative(-1, &page->_mapcount)) {
-#ifdef CONFIG_DEBUG_VM
if (unlikely(page_mapcount(page) < 0)) {
printk (KERN_EMERG "Eeek! page_mapcount(page) went negative! (%d)\n", page_mapcount(page));
printk (KERN_EMERG " page->flags = %lx\n", page->flags);
printk (KERN_EMERG " page->count = %x\n", page_count(page));
printk (KERN_EMERG " page->mapping = %p\n", page->mapping);
+ BUG();
}
-#endif
- BUG_ON(page_mapcount(page) < 0);
+
/*
* It would be tidy to reset the PageAnon mapping here,
* but that might overwrite a racing page_add_anon_rmap
--
SUSE Labs, Novell Inc.
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