On Wednesday 18 June 2008 17:29, KAMEZAWA Hiroyuki wrote: > 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. > Thanks Acked-by: Nick Piggin <npiggin@xxxxxxx> Andrew, this is a bugfix to mm-speculative-page-references.patch > 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
