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SubjectRe: [PATCH] fix double unlock_page() in 2.6.26-rc5-mm3 kernel BUG at mm/filemap.c:575!
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On Thu, 2008-06-12 at 19:13 -0700, Andrew Morton wrote:
> On Fri, 13 Jun 2008 10:44:44 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
> > This is reproducer of panic. "quick fix" is attached.
>
> Thanks - I put that in
> ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.26-rc5/2.6.26-rc5-mm3/hot-fixes/
>
> > But I think putback_lru_page() should be re-designed.
>
> Yes, it sounds that way.

Here's a proposed replacement patch that reworks putback_lru_page()
slightly and cleans up the call sites. I still want to balance the
get_page() in isolate_lru_page() with a put_page() in putback_lru_page()
for the primary users--vmscan and page migration. So, I need to drop
the lock before the put_page() when handed a page with null mapping and
a single reference count as the page will be freed on put_page() and a
locked page would bug out in free_pages_check()/bad_page().

Lee

PATCH fix page unlocking protocol for putback_lru_page()

Against: 2.6.26-rc5-mm3

Replaces Kame-san's hotfix:
fix-double-unlock_page-in-2626-rc5-mm3-kernel-bug-at-mm-filemapc-575.patch

Applies at end of vmscan/unevictable/mlock series to avoid patch conflicts.

1) modified putback_lru_page() to drop page lock only if both page_mapping()
NULL and page_count() == 1 [rather than VM_BUG_ON(page_count(page) != 1].
I want to balance the put_page() from isolate_lru_page() here for vmscan
and, e.g., page migration rather than requiring explicit checks of the
page_mapping() and explicit put_page() in these areas. However, the page
could be truncated while one of these subsystems holds it isolated from
the LRU. So, need to handle this case. Callers of putback_lru_page()
need to be aware of this and only call it with a page with NULL
page_mapping() when they will no longer reference the page afterwards.
This is the case for vmscan and page migration.

2) m[un]lock_vma_page() already will not be called for page with NULL
mapping. Added VM_BUG_ON() to assert this.

3) modified clear_page_lock() to skip the isolate/putback shuffle for
pages with NULL mapping, as they are being truncated/freed. Thus,
any future callers of clear_page_lock() need not be concerned about
the putback_lru_page() semantics for truncated pages.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>

mm/mlock.c | 29 +++++++++++++++++++----------
mm/vmscan.c | 12 +++++++-----
2 files changed, 26 insertions(+), 15 deletions(-)

Index: linux-2.6.26-rc5-mm3/mm/mlock.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/mm/mlock.c 2008-06-12 11:42:59.000000000 -0400
+++ linux-2.6.26-rc5-mm3/mm/mlock.c 2008-06-13 09:47:14.000000000 -0400
@@ -59,27 +59,33 @@ void __clear_page_mlock(struct page *pag

dec_zone_page_state(page, NR_MLOCK);
count_vm_event(NORECL_PGCLEARED);
- if (!isolate_lru_page(page)) {
- putback_lru_page(page);
- } else {
- /*
- * Page not on the LRU yet. Flush all pagevecs and retry.
- */
- lru_add_drain_all();
- if (!isolate_lru_page(page))
+ if (page->mapping) { /* truncated ? */
+ if (!isolate_lru_page(page)) {
putback_lru_page(page);
- else if (PageUnevictable(page))
- count_vm_event(NORECL_PGSTRANDED);
+ } else {
+ /*
+ * Page not on the LRU yet.
+ * Flush all pagevecs and retry.
+ */
+ lru_add_drain_all();
+ if (!isolate_lru_page(page))
+ putback_lru_page(page);
+ else if (PageUnevictable(page))
+ count_vm_event(NORECL_PGSTRANDED);
+ }
}
}

/*
* Mark page as mlocked if not already.
* If page on LRU, isolate and putback to move to unevictable list.
+ *
+ * Called with page locked and page_mapping() != NULL.
*/
void mlock_vma_page(struct page *page)
{
BUG_ON(!PageLocked(page));
+ VM_BUG_ON(!page_mapping(page));

if (!TestSetPageMlocked(page)) {
inc_zone_page_state(page, NR_MLOCK);
@@ -92,6 +98,8 @@ void mlock_vma_page(struct page *page)
/*
* called from munlock()/munmap() path with page supposedly on the LRU.
*
+ * Called with page locked and page_mapping() != NULL.
+ *
* Note: unlike mlock_vma_page(), we can't just clear the PageMlocked
* [in try_to_munlock()] and then attempt to isolate the page. We must
* isolate the page to keep others from messing with its unevictable
@@ -110,6 +118,7 @@ void mlock_vma_page(struct page *page)
static void munlock_vma_page(struct page *page)
{
BUG_ON(!PageLocked(page));
+ VM_BUG_ON(!page_mapping(page));

if (TestClearPageMlocked(page)) {
dec_zone_page_state(page, NR_MLOCK);
Index: linux-2.6.26-rc5-mm3/mm/vmscan.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/mm/vmscan.c 2008-06-12 11:39:09.000000000 -0400
+++ linux-2.6.26-rc5-mm3/mm/vmscan.c 2008-06-13 09:44:44.000000000 -0400
@@ -1,4 +1,4 @@
-/*
+ /*
* linux/mm/vmscan.c
*
* Copyright (C) 1991, 1992, 1993, 1994 Linus Torvalds
@@ -488,6 +488,9 @@ int remove_mapping(struct address_space
* lru_lock must not be held, interrupts must be enabled.
* Must be called with page locked.
*
+ * If page truncated [page_mapping() == NULL] and we hold the last reference,
+ * the page will be freed here. For vmscan and page migration.
+ *
* return 1 if page still locked [not truncated], else 0
*/
int putback_lru_page(struct page *page)
@@ -502,12 +505,11 @@ int putback_lru_page(struct page *page)
lru = !!TestClearPageActive(page);
was_unevictable = TestClearPageUnevictable(page); /* for page_evictable() */

- if (unlikely(!page->mapping)) {
+ if (unlikely(!page->mapping && page_count(page) == 1)) {
/*
- * page truncated. drop lock as put_page() will
- * free the page.
+ * page truncated and we hold last reference.
+ * drop lock as put_page() will free the page.
*/
- VM_BUG_ON(page_count(page) != 1);
unlock_page(page);
ret = 0;
} else if (page_evictable(page, NULL)) {



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