linux-kernel - Re: 2.6.18: Kernel BUG at mm/rmap.c:522

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Date:	Wed, 4 Oct 2006 16:39:35 -0400
From:	Dave Jones <davej@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andre Noll <maan@...temlinux.org>,
	Nick Piggin <nickpiggin@...oo.com.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, andrea@...e.de, riel@...hat.com
Subject: Re: 2.6.18: Kernel BUG at mm/rmap.c:522

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.

This patch brings it back unconditionally, and moves the BUG()
into the if arm.

Signed-off-by: Dave Jones <davej@...hat.com>

--- 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
-- 
http://www.codemonkey.org.uk
-
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