Re: [BUG] Bad page flags when process using mlock()ed memory exits

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Jun 19, 2009 at 02:11:21PM +1000, Peter Chubb wrote:
> 
> In recent kernels I've been seeing many mesages of the form:
> 
> BUG: Bad page state in process reiserfsck  pfn:79c58
> page:c3d03b00 flags:8050000c count:0 mapcount:0 mapping:(null) index:8095
> Pid: 3927, comm: reiserfsck Not tainted 2.6.30-test-05456-gda456f1 #60
> Call Trace:
>  [<c134a67c>] ? printk+0xf/0x13
>  [<c10774dc>] bad_page+0xc9/0xe2
>  [<c1078041>] free_hot_cold_page+0x5c/0x204
>  [<c1078206>] __pagevec_free+0x1d/0x25
>  [<c107ac3e>] release_pages+0x14e/0x18e)
>  [<c108ef8a>] free_pages_and_swap_cache+0x69/0x82
>  [<c1089458>] exit_mmap+0xf6/0x11f
>  [<c102afcd>] mmput+0x39/0xaf
>  [<c102e534>] exit_mm+0xe5/0xed
>  [<c102fa66>] do_exit+0x13f/0x578
>  [<c102fefd>] do_group_exit+0x5e/0x85
>  [<c102ff37>] sys_exit_group+0x13/0x17
>  [<c10031ef>] sysenter_do_call+0x12/0x3c
> Disabling lock debugging due to kernel taint
> 
> This appears to have been introduced by patch 
>     da456f14d2f2d7350f2b9440af79c85a34c7eed5
>     page allocator: do not disable interrupts in free_page_mlock()
> 
> That patch removed the free_page_mlock() from free_pages_check(), so
> if free_hot_cold_page() is called on an Mlocked page (e.g., if a
> process that used mlock() calls exit()) free_pages_check() will always
> barf, whereas before it would just unlock the page.

I prepared a fix, thanks for chasing it down.

Mel, to keep this simple I just used the atomic test-clear, but if I
am not mistaken we should not need any atomicity here, so we could
probably add a __TestClearPage version and use this instead...?

---
>From 493b74e8615db4e3323b5b169b0b8265dfd7c3f4 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Fri, 19 Jun 2009 19:30:56 +0200
Subject: [patch] mm: page_alloc: clear PG_locked before checking flags on free

da456f1 "page allocator: do not disable interrupts in free_page_mlock()" moved
the PG_mlocked clearing after the flag sanity checking which makes mlocked
pages always trigger 'bad page'.  Fix this by clearing the bit up front.

Reported-by: Peter Chubb <peter.chubb@nicta.com.au>
Debugged-by: Peter Chubb <peter.chubb@nicta.com.au>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mel@csn.ul.ie>
---
 mm/page_alloc.c |    9 ++++-----
 1 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6f0753f..30d5093 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -488,7 +488,6 @@ static inline void __free_one_page(struct page *page,
  */
 static inline void free_page_mlock(struct page *page)
 {
-	__ClearPageMlocked(page);
 	__dec_zone_page_state(page, NR_MLOCK);
 	__count_vm_event(UNEVICTABLE_MLOCKFREED);
 }
@@ -558,7 +557,7 @@ static void __free_pages_ok(struct page *page, unsigned int order)
 	unsigned long flags;
 	int i;
 	int bad = 0;
-	int clearMlocked = PageMlocked(page);
+	int wasMlocked = TestClearPageMlocked(page);
 
 	kmemcheck_free_shadow(page, order);
 
@@ -576,7 +575,7 @@ static void __free_pages_ok(struct page *page, unsigned int order)
 	kernel_map_pages(page, 1 << order, 0);
 
 	local_irq_save(flags);
-	if (unlikely(clearMlocked))
+	if (unlikely(wasMlocked))
 		free_page_mlock(page);
 	__count_vm_events(PGFREE, 1 << order);
 	free_one_page(page_zone(page), page, order,
@@ -1022,7 +1021,7 @@ static void free_hot_cold_page(struct page *page, int cold)
 	struct zone *zone = page_zone(page);
 	struct per_cpu_pages *pcp;
 	unsigned long flags;
-	int clearMlocked = PageMlocked(page);
+	int wasMlocked = TestClearPageMlocked(page);
 
 	kmemcheck_free_shadow(page, 0);
 
@@ -1041,7 +1040,7 @@ static void free_hot_cold_page(struct page *page, int cold)
 	pcp = &zone_pcp(zone, get_cpu())->pcp;
 	set_page_private(page, get_pageblock_migratetype(page));
 	local_irq_save(flags);
-	if (unlikely(clearMlocked))
+	if (unlikely(wasMlocked))
 		free_page_mlock(page);
 	__count_vm_event(PGFREE);
 
-- 
1.6.3