In speculative page cache look up protocol, page_count(page) is set to 0 while radix-tree modification is going on, truncation, migration, etc... While page migration, a page fault to page under migration does - look up page table - find it is migration_entry_pte - decode pfn from migration_entry_pte and get page of pfn_page(pfn) - wait until page is unlocked It does get_page() -> wait_on_page_locked() -> put_page() now. In page migration's radix-tree replacement, page_freeze_refs() -> page_unfreeze_refs() is called. And page_count(page) turns to be zero and must be kept to be zero while radix-tree replacement. If get_page() is called against a page under radix-tree replacement, the kernel panics(). To avoid this, we shouldn't increment page_count() if it is zero. This patch uses get_page_unless_zero(). Even if get_page_unless_zero() fails, the caller just retries. But will be a bit busier. Change log v1->v2: - rewrote the patch description and added comments. From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> --- mm/migrate.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) 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 @@ -242,8 +242,15 @@ void migration_entry_wait(struct mm_stru goto out; page = migration_entry_to_page(entry); - - get_page(page); + /* + * Once radix-tree replacement of page migration started, page_count + * *must* be zero. And, we don't want to call wait_on_page_locked() + * against a page without get_page(). + * So, we use get_page_unless_zero(), here. Even failed, page fault + * will occur again. + */ + if (!get_page_unless_zero(page)) + goto out; pte_unmap_unlock(ptep, ptl); wait_on_page_locked(page); put_page(page); -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
