Re: [PATCH][RFC] fix kernel BUG at mm/migrate.c:719! in2.6.26-rc5-mm3

[KAMEZAWA Hiroyuki]

On Tue, 17 Jun 2008 16:35:01 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> This patch also fixes a race between migrate_entry_wait and
> page_freeze_refs in migrate_page_move_mapping.
> 
Ok, let's fix one by one. please add your Signed-off-by if ok.

This is a fix for page migration under speculative page lookup protocol.
-Kame
==
In speculative page cache lookup protocol, page_count(page) is set to 0
while radix-tree midification is going on, truncation, migration, etc...

While page migration, a page fault to page under migration should wait
unlock_page() and migration_entry_wait() waits for the page from its
pte entry. It does get_page() -> wait_on_page_locked() -> put_page() now.

In page migration, page_freeze_refs() -> page_unfreeze_refs() is called.

Here, page_unfreeze_refs() expects page_count(page) == 0 and panics
if page_count(page) != 0. To avoid this, we shouldn't touch page_count()
if it is zero. This patch uses page_cache_get_speculative() to avoid
the panic.

From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/migrate.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: test-2.6.26-rc5-mm3/mm/migrate.c
===================================================================
--- test-2.6.26-rc5-mm3.orig/mm/migrate.c
+++ test-2.6.26-rc5-mm3/mm/migrate.c
@@ -243,7 +243,8 @@ void migration_entry_wait(struct mm_stru
 
 	page = migration_entry_to_page(entry);
 
-	get_page(page);
+	if (!page_cache_get_speculative())
+		goto out;
 	pte_unmap_unlock(ptep, ptl);
 	wait_on_page_locked(page);
 	put_page(page);


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