linux-kernel - BUG?: PAGE_FLAGS_CHECK_AT_PREP seems to be cleared too early (Was Re: I just got got another Oops

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Message-Id: <20090316170359.858e7a4e.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 16 Mar 2009 17:03:59 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"hugh@...itas.com" <hugh@...itas.com>
Cc:	Gene Heskett <gene.heskett@...il.com>,
	David Newall <davidn@...idnewall.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: BUG?: PAGE_FLAGS_CHECK_AT_PREP seems to be cleared too early (Was
 Re: I just got got another Oops

Hi,
I'm sorry if I miss something..

>From this patch
==
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=79f4b7bf393e67bbffec807cc68caaefc72b82ee
==
#define PAGE_FLAGS_CHECK_AT_PREP       ((1 << NR_PAGEFLAGS) - 1)
...
@@ -468,16 +467,16 @@ static inline int free_pages_check(struct page *page)
                (page_count(page) != 0)  |
                (page->flags & PAGE_FLAGS_CHECK_AT_FREE)))
....
+       if (PageReserved(page))
+               return 1;
+       if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
+               page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
+       return 0;
 }
==

PAGE_FLAGS_CHECK_AT_PREP is cleared by free_pages_check().

This means PG_head/PG_tail(PG_compound) flags are cleared here and Compound page
will never be freed in sane way.

Regards,
-Kame

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