On Wed, Sep 28, 2011 at 10:14:52AM +0200, Johannes Weiner wrote: > On Wed, Sep 28, 2011 at 10:45:30AM +0900, Minchan Kim wrote: > > When racing between putback_lru_page and shmem_unlock happens, > > progrom execution order is as follows, but clear_bit in processor #1 > > could be reordered right before spin_unlock of processor #1. > > Then, the page would be stranded on the unevictable list. > > > > spin_lock > > SetPageLRU > > spin_unlock > > clear_bit(AS_UNEVICTABLE) > > spin_lock > > if PageLRU() > > if !test_bit(AS_UNEVICTABLE) > > move evictable list > > smp_mb > > if !test_bit(AS_UNEVICTABLE) > > move evictable list > > spin_unlock > > > > But, pagevec_lookup in scan_mapping_unevictable_pages has rcu_read_[un]lock so > > it could protect reordering before reaching test_bit(AS_UNEVICTABLE) on processor #1 > > so this problem never happens. But it's a unexpected side effect and we should > > solve this problem properly. > > > > This patch adds a barrier after mapping_clear_unevictable. > > > > side-note: I didn't meet this problem but just found during review. > > > > Cc: Johannes Weiner <jweiner@xxxxxxxxxx> > > Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> > > Cc: Mel Gorman <mel@xxxxxxxxx> > > Cc: Rik van Riel <riel@xxxxxxxxxx> > > Cc: Lee Schermerhorn <lee.schermerhorn@xxxxxx> > > Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx> > > --- > > mm/shmem.c | 1 + > > mm/vmscan.c | 11 ++++++----- > > 2 files changed, 7 insertions(+), 5 deletions(-) > > > > diff --git a/mm/shmem.c b/mm/shmem.c > > index 2d35772..22cb349 100644 > > --- a/mm/shmem.c > > +++ b/mm/shmem.c > > @@ -1068,6 +1068,7 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user) > > user_shm_unlock(inode->i_size, user); > > info->flags &= ~VM_LOCKED; > > mapping_clear_unevictable(file->f_mapping); > > + smp_mb__after_clear_bit(); > > scan_mapping_unevictable_pages(file->f_mapping); > > I always get nervous when I see undocumented barriers. Maybe add a > teensy tiny comment here? Agree. I will try it. > > /* > * Ensure that a racing putback_lru_page() can see > * the pages of this mapping are evictable when we > * skip them due to !PageLRU during the scan. > */ > > Or something like that. Otherwise, nice catch :-) > > Acked-by: Johannes Weiner <jweiner@xxxxxxxxxx> Thanks! -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>