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    SubjectRe: [PATCH][RFC] fix kernel BUG at mm/migrate.c:719! in 2.6.26-rc5-mm3
    On Wed, 18 Jun 2008 10:13:49 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > On Tue, 17 Jun 2008 16:35:01 +0900
    > Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> 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.
    >
    Agree. It should be fixed independently.

    Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

    > 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@mxp.nes.nec.co.jp>
    > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > ---
    > 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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